To better synchronize the downtime in both recipes, start making the dark chocolate mousse first and chill it in the fridge, while you make the chocolate sheet cake.
 

Chocolate Mousse

• In the bowl of a stand mixer fitted with the whisk attachment, whip 200 grams cold heavy cream, 100 grams mascarpone, and one teaspoon vanilla bean paste on medium speed until soft peaks form, about 2 minutes. Cover and chill until needed.
   
• If using coffee in the chocolate mousse, dissolve 1 tablespoon of espresso powder in 1 tablespoon of boiling water, give it a whisk, and set aside to cool down.
   
• Bakers Note; Warm eggs ( in the shell ) by placing them in a bowl of hot tap water for about 5 minutes before you separate the egg whites from the yolks. This step will whip the egg mixture into a fluffy yellow cloud and give you lots of volumes.
   
• Add 2 large eggs ( 100 grams) and 6 large egg yolks (approx 100 up to 120 grams)  into the stand mixer bowl fitted with a whisk attachment. Split one vanilla bean lengthwise, and use the tip of a sharp knife to scrape out the seeds. Add the vanilla seeds to the bowl with the egg/egg yolks. Whisk on medium-high speed for 10 minutes. Use a timer!
   
• Meanwhile, in a small saucepan, combine 100 grams superfine sugar, 50-gram glucose, and one tablespoon of water (15 grams); using a small spatula, stir until all of the sugar is moistened. Heat over medium-high heat, constantly stirring, until the sugar dissolves and the syrup begins to boil around the edges. Stop stirring and cook until the entire surface is covered with large bubbles and you clearly see the saucepan's bottom. This takes just a few minutes.  
   
• As soon as the sugar syrup is done, turn off the stand mixer and pour one-third of the sugar syrup mixture over the egg/yolk mixture; immediately beat on high speed for 5 seconds. Stop the mixer again, add one-third of the sugar syrup, and beat on high speed for another 5 seconds. Stop the mixer again and continue with the last one-third of the sugar syrup mixture; mix for 5 seconds more.  Then continue to whisk for 10 minutes more. Use a timer!
   
• Meanwhile, put 200 grams of dark chocolate, 66 to 70%, in a heatproof bowl over a pan of simmering water ( bain-marie) and leave to melt slowly. Remove the bowl from the hot water before it is completely melted, and then stir until it is melted and smooth. 
   
• Bakers Note; The melted chocolate must cool a bit ( 45C° /113F° is the ideal temperature) so that it doesn't seize and clump when you fold the melted chocolate into the egg mixture. About 5 minutes. 
   
• Using a balloon whisk, carefully fold one-third of the whipped egg/yolk mixture into the melted chocolate. Repeat twice. Use a spatula to scrape the side and the bottom of the bowl.
   
• Pull the whipped mascarpone cream from the refrigerator and give it a couple of rounds with the whisk to bring it back to soft peaks, if needed.
   
• Add half of the softly whipped mascarpone cream into the chocolate mixture, and combine with a whisk. Add the rest of the whipped cream and combine until smooth. Use a silicon spatula to scrape the bowl's side and bottom. If using coffee, add one or two tablespoons of slightly cooled coffee and combine.
   
• Place the chocolate mousse in the fridge for at least 1 hour (or longer if needed) to firm up.

Chocolate Cake
 

• Bakers Note for the cake; Make sure the butter, and eggs, are at room temperature; the ideal temperature is about 22C°/ 71.6F°Allow the butter to sit out on the counter for about 1 to 2 hours before baking this cake. The amount of time depends on-off the course, the weather, and how cool you keep your kitchen. To test if your butter is at room temp, poke it with your finger. Your finger should make an indent without sinking into the butter. The butter should not be shiny or greasy. It will be cool to the touch but not warm. If it feels greasy, place it back into the fridge for some minutes to firm up.
   
• Warm the eggs ( in the shell) by placing the eggs in a bowl of hot tap water for about 5 minutes before adding them to the butter mixture.
   
• To make the chocolate sheet cake, adjust the oven rack to the lower rack of the oven and preheat the oven to 165C° /  325F° 30 minutes before baking.
   
• Bakers Note; All the ingredients, →, unsalted butter, eggs, and sour cream, need to be at room temperature. If the ingredients are too cold, they won't emulsify or whip up with enough air to create a good rise in your chocolate sheet cake. 
   
• Use a pastry brush to lightly coat the inside of 23 x 32 cm with a 5 cm tall sides baking tray (9 x 13 inches x 2 inches) with softened butter and line it with parchment paper. Set aside for now.
   
• In a medium bowl, sift together 275-gram all-purpose flour, 85-gram potato starch, two teaspoons baking powder, one teaspoon baking soda, and 1/2 teaspoon fine sea salt. Give it a quick whisk to combine and set aside for now.
   
• Bakers Note; Don't be leery about the addition of coffee to this cake; it only helps deepen and intensify the chocolate flavor. You won't taste the coffee at all.
   
• In a medium bowl, add 130-gram unsweetened cocoa powder and 245 grams warm coffee or espresso, and give it a quick whisk until 100% smooth. The temperature of the mixture will now be around (57C°/134.F°); add 300-gram sour cream and give it a whisk until combined and smooth. The temperature of the mixture will now be around 30C°/86F°. Set aside, for now, to cool down to 22C°/ 71.6F°.
   
• Place 255-gram salted or unsalted butter ( diced) and two teaspoons of vanilla bean paste in the bowl of a stand mixer fitted with the paddle or whisk attachment. Turn to medium-low speed and cream the butter until fluffy, stopping to scrape down the sides and bottom of the bowl as needed, about 5 minutes. 
   
• Add the 575-gram superfine sugar, and mix fluffy for about 5 minutes. Stopping to scrape down the sides and bottom of the bowl as needed.
   
• Bakers note; make sure to mix on medium-low speed and never on high speed. If you mix too fast or too long, you will collapse all your air bubbles, and your chocolate sheet cake will be really dense and brick-like-not light and fluffy.
   
• Scrape down the sides and bottom of the bowl, and add the five large eggs one at a time, mixing slowly on medium speed after each addition, about 30 seconds. It´s important to add them one at a time to be completely incorporated into the batter before adding in the next.
   
•  At this stage, the mixture may look broken, which is normal at this stage ( overwhipping the eggs could cause the cake to expand too much during baking and then deflate)
   
• Bakers note; When you add the flour/cocoa mixture at this stage, it´s always important to mix as slowly and as little as necessary since you don't want to traumatize your batter.
   
• Add one-third of the flour mixture and one-third of the cocoa, coffee- and sour cream mixture to the butter mixture, beating at low speed until just combined—about 20 seconds. Stop scraping down the bowl as needed. Repeat this step twice.
   
• Remove the bowl from the mixer stand and scrape the bottom of the bowl to incorporate any dry ingredients that may have settled there. If you still have lumps left and if you have an immersion blender, emulsify with a hand blender until the cake batter is smooth. ( just a few seconds)
   
• Pour the chocolate cake batter into your prepared baking tray, spreading it evenly with an offset spatula to reach into the corners.  Tap the baking tray on the kitchen a few times to release excess air bubbles.
   
• Bake until a cake tester/ toothpick is inserted in the center of the chocolate cake and will emerge with a few crumbs still attached. It takes about 35 to 45 minutes. 
   
• Let the cake cool in the pan until no trace of warmth remains, about 60 minutes. 
   
• Carefully turn the chocolate sheet cake out onto a wire rack; peel off the parchment paper and let it cool completely, or leave it in the baking tray if you prefer that.
   
• Once cool, wrap the cake tightly in plastic wrap at room temperature until you are ready to spread the chocolate mousse on top.
   
• Bakers Note; The chocolate mousse looks very airy when you spread it onto the chocolate cake. But will look very beautiful and glossy after it gets set on the cake. Your chocolate mousse is also lighter in color when you spread it but will also darken after it sits out a bit.
   
• Remove the chocolate mousse from the fridge, and swoop all of the chocolate mousse all over the chocolate sheet cake in a thick, even layer. Decorate with sprinkles etc.
   
• Slice into desired pieces ( 24 pieces) and serve immediately or store in the fridge ( covered) for up to 1 week. The texture of the chocolate sheet cake is light, tender, and moist and stays like that for at least 7 days.
   
• Remove the chilled cake from the fridge 20 to 30 minutes before serving and let it sit at room temperature until ready to serve.