Servings: enough frosting to frost a 3-layer cake

1½ C  butter, softened 1½ C  butter, softened
8 oz cream cheese, softened 8 oz cream cheese, softened
1½ C  unsweetened cocoa powder 1½ C  unsweetened cocoa powder
3 t  vanilla extract 3 t  vanilla extract
7-8 C  powdered sugar 7 to 8 C  powdered sugar
about ¼ C  milk (as needed) about ¼ C  milk (as needed)

In a large bowl, beat together butter and cream cheese until fluffy. Use a hand mixer or stand mixer for best results
Add in cocoa powder and vanilla extract. Beat until combined.
Beat in powdered sugar, 1 cup at a time. Add milk as necessary to make a spreadable consistency. The frosting should be very thick and will thicken even more if refrigerated.