Source: Joy of Cooking
Have all ingredients at room temperature. Preheat oven to 350 degrees F.
6 tablespoons butter
1 cup granulated sugar
1 cup sourdough starter
3/4 cup milk
3 ounces melted semisweet baking chocolate
1 teaspoon vanilla extract
1 3/4 cups sifted all-purpose flour or cake flour
1 teaspoon baking soda
1/2 teaspoon salt\
Cream thoroughly the butter and sugar. Add and beat the eggs. Stir in sourdough starter, milk, baking chocolate and vanilla extract.
Sift together the flour, baking powder and salt. Fold the flour mixture into the batter and stir until smooth. Pour into two greased 1 1/2 x 8-inch round cake pans, or one 9-inch square cake pan. Bake for about 40 minutes for one square pan or 25 minutes for two round pans, or until a cake tester comes out clean.