For the smoothest chocolate buttercream, I recommend using a food processor. If you don't have one, a bowl and paddle attachment on your mixer will work fine. Start by whipping the semi-cold butter until it's creamy. Then, mix in the powdered sugar, espresso powder, and vanilla if you're adding it. Next, carefully blend in the melted chocolate. You'll still end up with a delicious buttercream.
 

 Silky Chocolate Buttercream
 

• For this recipe do not use chocolate chips, Which are designed NOT to melt so they can hold their shape in chocolate cookies and will not work in this recipe.  The chocolate I am referring to is a good quality semisweet dark and pure chocolate, I use 66% semisweet chocolate callets from Valhrona, Callebaut 70%, or Lindt chocolate is also a good brand to use for this recipe.
   
• Bakers Note; "For the best results with my silky smooth chocolate buttercream recipe, it's important to use high-quality salted butter. This is because some brands of salted butter may have a higher water content, which can negatively impact the texture and richness of your buttercream. If you can't find a good quality salted butter, opt for unsalted butter instead and add a pinch of salt when mixing the butter. This small adjustment ensures your chocolate buttercream has the perfect balance of flavor and texture."
   
•  Take your butter out of the fridge for about 20 minutes before making your chocolate buttercream. "For this recipe, you want to use semi-cold butter. Semi-cold butter needs to be pliable but still cool. It should be around 16°C/60.8°F."
   
• Bakers Note; When you're melting dark chocolate for the buttercream in the next step, go low and slow with the heat. Take it off the burner just before all the pieces are completely melted. Stirring the chocolate after you remove it from the heat will help the last bits melt without getting too hot. This way, you'll get smooth, creamy chocolate that's just right for mixing into your buttercream."
   
• In a medium heatproof bowl set over barely simmering water, melt 150-gram bittersweet chocolate, occasionally stirring until the chocolate is almost melted! Remove the chocolate from the heat and, with the silicone spatula, stir until fully melted. Allow it to cool until it is no longer warm to the touch but is still fluid. 
   
• Sift 175 grams of powdered sugar and place into your food processor, then add 180 grams of semi-cold salted or unsalted butter ( cut into small cubes), 1 teaspoon of instant espresso powder, and 1 teaspoon of vanilla bean paste ( optional) into a food processor and pulse on high speed until smooth, about 1 to 2 minutes. Be careful not to overmix the butter. Overmixing can make it too soft, and you end up with a very loose buttercream. 
   
• Scrape down the sides and bottom of your bowl; add the melted and slightly cooled chocolate to the food processor, and pulse on high speed until silky smooth, about 1 minute! Again don't mix more than needed.
   
• Once the chocolate buttercream is silky smooth and lump-free, Add the 40-gram cold heavy cream and blend at high speed for 1 minute more! It is now ready!
   
• Your chocolate buttercream is now ready to use as a filling in your cakes or decorate your cakes with!
  
   
• Bakers Note; "If your silky smooth chocolate buttercream is too soft at this stage, it might be because the butter was overwhipped or the chocolate was a touch too warm. But don't worry—it's an easy fix. if the chocolate buttercream turns out soft, you can firm it up by placing it in the fridge. Remember to check every five minutes because high-quality dark chocolate firms up fast. Once it reaches a better consistency, whip it again, either in your food processor or with a paddle attachment in a bowl, until it's just how you like it."
   
• You can use this frosting to fill or cover your cakes, and it is firm enough after a few minutes in the fridge to pipe onto your cupcakes. 
   
• For a fluffier buttercream frosting, In the bowl of an electric mixer fitted with the paddle attachment, add the chocolate frosting, and beat at low-medium speed until more light in color and fluffy. The longer you whip, You´ll notice the chocolate frosting lightens in color quite a bit as you beat it, but it will also darken after it sits out on your cake or cupcakes.
   
•  If not used right away, store it in an airtight container in the refrigerator for up to 3 days. When ready to use, bring it to room temperature and pulse it in your food processor to bring it back to a spreadable consistency, or use your stand mixer and whisk attachment.
   
• If you decorate/fill your cake with this frosting and store it in the refrigerator, allow the cake to stand at room temperature for at least 2 hours before serving! I prefer serving this frosting on my cake without being in the fridge for the yummiest and silkiest feel in the mouth!