Preheat the oven to 375 degrees. Grease and flour two, 8-inch round cake layer pans.
To the melted chocolate add the 1 tablespoon sugar, hot water and baking soda; blend together. Cool.
Sift the flour once, measure, add the baking powder and the salt, and sift together 3 times.
Cream the butter and the sugar together until light and fluffy. Add the flour mixture alternately with the milk, a small amount at a time, beating after each addition until smooth. Add vanilla. Beat the egg whites until they will hold up in moist peaks. Stir them quickly, but thoroughly into the cake batter. Reserve one third of the cake batter and add the chocolate mixture to the reserved batter. Put by tablespoons into the prepared pans, alternately the light and dark mixtures. Then, with a knife, cut carefully through the batter one time in a wide, zigzag course. Bake for 25 minutes, or until a toothpick inserted in the middle of the cakes comes out clean. Cool for 10 minutes in the pans and then turn out onto wire racks to cool completely. Frost as desired.
Source: “Homemaker’s Cookbook-Cakes”, The Progressive Farmer, 1971