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, until blended
well.
Mix soda and sour cream and whisk into chocolate mixture.
Mix flour and baking powder well, and add to batter, mixing thoroughly.
Beat egg whites until stiff but not dry. Stir in a quarter of the egg whites
thoroughly into the batter. Scoop the remaining egg whites on top of the batter
and gently fold in.
Pour batter into prepared pan. Set on the middle rack of the oven and bake
for 40-50 min or until the edges have pulled away from the pan and the cake
tester inserted comes out clean.
Cool in pan for 10 minutes, remove and cool completely before frosting.
Chocolate Frosting
2 tbsp sweet butter
¾ cup semisweet chocolate chips
6 tbsp heavy cream
1¼ cup confectioners sugar
1 tspn vanilla
Place all ingredients in a heavy saucepan and whisk until smooth over low
heat.