Cream butter and sugar. Stir in egg yolk.
Stir the soda into the sour cream.
Sift the dry ingredients together twice.
Add all ingredients to the butter-sugar mixture in the order given.
Bake in 2 buttered and lightly floured layer-cake tins about 20 to 25 minutes in a preheated 350° oven. When cool, spread top of one layer with the filling:
Mix filling ingredients in order given and cook 20 minutes, stirring often so mixture will not scorch.
Center plain layer over filled layer and frost top and sides with frosting:
Boil sugars and water until they spin a thread. Pour slowly over the beated egg whites. Frost the cake.
Melt 2 ounces (2 squares) of unsweetened chocolate over hot water. Pour over cake and let drip down the sides.