In a the bowl of an electric stand mixer, fitted with whisk attachment , whip butter on moderately high speed until very pale and fluffy , about 6 - 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. Increase mixer to medium-high speed and whip mixture until pale and very fluffy about 4 - 5 minutes, occasionally scraping down sides of the bowl.
