Heat oven to 350℉.
Lightly dust two greased 8-inch springform pans with cocoa powder. In a saucepan, heat butter, beer, and cocoa powder until butter melts. Let cool.
Sift dry ingredients together. Add the beer-cocoa mixture and beat for 1 minute on medium speed.
Add the eggs and sour cream and beat 2 minutes on medium speed. Pour batter into prepared pans and bake for 25 to 30 minutes, or until a toothpick inserted into the middle comes out clean.
Place pans on a wire rack, cool for 10 minutes, remove sides, and cool completely.
Use a long serrated knife to even the tops of the cakes. Spread each layer with a thin coating of chocolate frosting, stack, and cover the sides with frosting.