I've been making this for years, from a cookbook my Mother gave me called The Pleasures of Afternoon Tea, from 1987, by Angela Hynes.
2 cups self-rising flour
1/2 cup butter or margarine, room temperature
3/4 cup sugar
1 tsp. vanilla extract
4 tablespoons milk
2 tablespoons unsweetened cocoa powder
2 tablespoons water
Preheat oven to 350 F. Grease a round, 9-inch diameter cake pan. Sift flour into a medium-size bowl; set aside. Cream butter and sugar with an electric mixer or a wooden spoon until light and fluffy. Beat in eggs 1 at a time; beat in vanilla. Alternately add flour and milk, beating until smooth after each addition. Pour half of batter into another bowl. Stir together cocoa and water; beat into 1 bowl of batter. Add alternate spoonfuls of light and chocolate batter to cake pan; gently swirl together with a fork. Bake 35 - 40 minutes. Cool on wire rack.
This is small but easy and tasty.