Cream butter and sugar
While butter creams, sift flour, salt and baking powder together.
Beat egg whites till stiff either while butter continues to cream, or in a separate bowl.
Melt chocolate in a double boiler. Stir in sugar and baking soda. When well melted, stir in boiling water.
Fold in the stiff egg whites.
Divide the batter into two portions, the white slightly larger than what will be the chocolate.
Add the chocolate mixture to the smaller batter portion and mix well.
Alternate adding globs of chocolate and vanilla batter to a greased and floured cake pan.
Bake in a preheated 350 degree oven for about 40 minutes. Wait for the smell of the baking cake to permeate the kitchen, then start checking for doneness. Cake should just start pulling away from the sides of the pan, and a cake tester should come out with crumbs, not clean. If clean, it's over baked.
I divide the milk into equal portions and divide the vanilla between them. Then I add 1/3 of the flour, mix it in, one half of the milk, mix, 1/3 of the flour, mix, last half of the milk, mix, last portion of flour and mix. All mixing is done by hand.
The milk will not mix right in.