Preheat the oven to 325 degrees F. Grease and flour a 10-inch Bundt pan.
Using an electric mixer, cream together the butter, sour cream, and sugar. Add the eggs, 2 at a time, beating well after each addition. Add the vanilla.
In another bowl, stir together the flour, baking powder, and cocoa. Add 1/2 the flour mixture to the creamed mixture, beat well, add the remaining 1/2 flour mixture, and continue to beat at medium speed for 2 minutes. Pour the batter into the prepared pan and bake for 1 hour 15 minutes. Continue to bake for an additional 15 minutes if necessary, but do not open the oven to check the cake for at least 1 hour.