Perfectly rich and decadent, silky smooth, brimming with chocolate flavor and the perfect finish to any chocolate, yellow or vanilla cake. Makes enough frosting for a 2-layer cake or about 24 cupcakes (unless you like a tower of frosting).
In a the bowl of an electric stand mixer, fitted with paddle attachment, whip butter on moderately high speed until very pale and fluffy (nearly white in color), about 5 - 8 minutes, occasionally scraping down sides of the bowl.
Sift in cocoa powder then add in powdered sugar, 3 Tbsp heavy cream and vanilla and mix on low speed until combine, scrapping down the sides of the bowl as needed and adding up to 1 tbsp additional cream to thin if desired.
For added fluffiness if desired continue to whip about 3 - 4 minutes longer.
Frosting can be made a few days in advance. Store in refrigerator in an airtight container. Bring to room temperature before using. If desired rewhip a little for added fluffiness.