In large bowl, beat 1/4 cup butter, 1/2 cup peanut butter and sugar with an electric mixer until light and fluffy. Add eggs, sour cream and 1 teaspoon vanilla; beat until well combined. Alternately add flour mixture and 2/3 cup plus 1 tablespoon milk to bowl, beating until just combined. Remove 2-1/2 cups batter from bowl and set aside. Beat in remaining 1 tablespoon milk and 2 tablespoons cocoa powder to remaining batter.