I recommend you use a mixer to beat the frosting. You can do it by hand. However, using a mixer causes the frosting to be lighter and fluffier than it will if done by hand.
8-1/3 Tablespoons butter, softened
9 Tablespoons unsweetened cocoa powder
3 cups powdered sugar
1/2 cup sour cream
2 teaspoons vanilla extract
Mix the butter and the cocoa powder.
Add the powdered sugar and stir in.
Stir in the sour cream and vanilla extract.
With a mixer, beat everything until it is smooth and spreadable.
This will frost a 13″x9″ cake or will frost and fill a 8″ or 9″ double layer cake.