In a large bowl, beat the butter on medium speed with an electric mixer, until creamy and smooth.
Add half of the powdered sugar and mix on low speed, then add the milk and vanilla. Gradually add the remaining powdered sugar until smooth, adjusting the milk to reach your desired consistency.
Add Cocoa Powder. Gradually add the black cocoa powder, beating on low speed until incorporated.
Once everything is combined, increase the mixer to high speed for about 2 minutes. If the frosting is too thick, add a splash of milk; if too thin, add a bit more powdered sugar until you reach the perfect consistency. Fill a piping bag and decorate cupcakes or frost a cake.
Notes
Enough to frost 22-24 cupcakes, a 9"x13" cake or layered cake.Storage and Freezing:
Store in an Airtight Container: Keep any leftover frosting in an airtight container in the refrigerator for up to one week.
Freezing Option: This black buttercream recipe freezes well! Place it in a freezer-safe bag or container, squeezing out as much air as possible, and freeze for up to 3 months. Thaw in the refrigerator before re-whipping to restore its creamy consistency.
For a Rich Chocolate Flavored Frosting:
1 cup unsalted butter, softened to room temperature