Sour MIlk Chocolate Cake

3 Tbsp. cocoa
4 Tbsp. butter (1/4 cup)
1 cup sugar
1 egg yolk
1 cup sour milk
1 1/4 cup flour
1 tsp. soda
1 tsp. vanilla

Dissolve cocoa in melted butter; add yolk of egg and sugar, beating well; add part of milk, flour sifted with soda, then rest of milk and vanilla. Bake in a moderate oven.