Preheat oven to 350 degrees. Cream butter and sugar until fluffy. Add yolks, incorporating well.
Sift together flour and baking powder. On low speed add flour to butter mixture alternating with sour cream and half and half. In a clean bowl, whip whites until stiff. Gradually add sugar and whip until glossy. Fold into batter. In separate bowl stir together cocoa, coffee and baking soda. Take off 1/3 of the batter and stir in cocoa mixture. Drop spoonfuls of each colored batter into 6 nonstick mini bundt cake pans. Bake at 350 for 35 to 40 minutes or until firm to the touch. Let cool in pan then turn out. Serve with warm chocolate sauce and banana-walnut ice cream