Ingredients
1/2 cup butter, softened1-1/4 cups packed brown sugar1 large egg, room temperature1 teaspoon vanilla extract1-1/2 cups all-purpose flour6 tablespoons baking cocoa1-1/2 teaspoons cream of tartar1 teaspoon baking soda1 teaspoon baking powder1/4 teaspoon salt3/4 cup water1/2 cup milkFROSTING:1/2 cup butter, softened1 cup confectioners’ sugar1/4 cup baking cocoa1 to 2 tablespoons milk1 can (16 ounces) vanilla frosting
Preparation
Grease a 13x9-in. baking pan; line with parchment. Grease the paper; set aside. In a large bowl, cream butter and brown sugar. Beat in egg and vanilla. Combine the flour, cocoa, cream of tartar, baking soda, baking powder and salt; add to creamed mixture alternately with water and milk, beating well after each addition.
Pour into prepared pan. Bake at 350° for 22-27 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before inverting onto a wire rack. Discard parchment. Cool cake completely.
For frosting, in a large bowl, beat the butter, confectioners’ sugar and cocoa until smooth. Beat in enough milk to achieve spreading consistency. Transfer cake to a serving platter. Spread with chocolate frosting; decorate with vanilla frosting.
