In a large bowl with an electric mixer on low speed beat sugar and butter or margarine until blended. Increase speed to high and beat until light and fluffy. Add flour, milk, baking powder, vanilla, salt and eggs; beat at medium speed until well mixed. Increase speed to high and beat batter 4 minutes longer.
Remove about 2 1/2 cups batter to a medium bowl. With a wire whisk or fork beat cocoa into batter in medium bowl until well blended.
Grease a 25cm ring tin. Alternately spoon vanilla and chocolate layers into prepared pan. With a large spoon, cut and twist through batters to obtain a marbled effect.
Bake at 180 degrees C for 1 hour or until a toothpick inserted in the centre of the cake comes out clean. Cool cake in pan and on a wire rack for 10 minutes. Remove cake from pan and cool completely.
Very easy, and very tasty! Looks impressive too. I loved it, and si dod everyone else who tried it. I have just got a second on the oven that I'm baking as a birthday cake for a friend. I covered my first attempt in a chocolate icing (Hershy's recipe, worked a treat!) but it really doesn't need it. - 17 Oct 2014