Sinners' Chocolate Cake

This is another recipe from the CASA Cookbook. So rich and delicious!!!

Cake:
1 cup boiling water
3 oz baking chocolate, unsweetened
1 stick unsalted butter
1 t vanilla
2 cups sugar
2 eggs, separated
1 t baking soda
1/2 cup sour cream
2 cups flour, sifted, plus 3 T
1 t baking powder

Frosting:
1/2 cup butter
4 oz baking chocolate, unsweetened
1/2 cup sour cream
3 cups powdered sugar
1 t vanilla
1-2 T milk, if necessary

Pour boiling water over chocolate and butter. Let stand until melted. Stir in vanilla and sugar, then whisk in egg yolks, one at a time, continuing to stir until mixture is smooth. Mix baking soda and sour cream and whisk into the chocolate mixture. Sift flour and baking powder together and add to batter, mixing thoroughly.
Beat egg whites until stiff but not dry. Stir a little of the whites thoroughly into the batter, then gently fold in the rest. Pour batter into 2 10" round, buttered and floured cake pans. Bake at 350 degrees for 25-30 minutes until toothpick comes out clean. Cool in pans 15 minutes, then remove from pan. Cool completely before frosting.
To make frosting, melt butter and chocolate together over low heat. Stir in rest of ingredients. Thin with milk if necessary.