"I first made this cake in my junior high home economics class. I've changed the recipe through the years to make the cake a little richer, and the icing is my own invention. --Katarina Greer, Victoria, British Columbia"
Grease a 13-in. x 9-in. x 2-in. baking pan; line with parchment paper. Grease the paper; set aside. In a mixing bowl, cream butter and brown sugar. Beat in egg and vanilla. Combine water and milk. Combine the flour, cocoa, cream of tartar, baking soda, baking powder and salt; add to creamed mixture alternately with milk mixture.
Pour into prepared pan. Bake at 350 degrees F for 22-27 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before inverting onto a wire rack. Remove and discard parchment paper. Cool cake completely.
For frosting, in a mixing bowl, cream butter, confectioners' sugar and cocoa until smooth. Beat in enough milk to achieve spreading consistency. Transfer cake to a serving platter or covered board. Spread with chocolate frosting; decorate with vanilla frosting.