Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9 inch round pan.

Place flour, baking powder, salt, splenda, margarine, eggs, vanilla, mashed potato, and milk into mixing bowl. Beat slowly to moisten, then beat with an electric mixer at medium speed for about 2 minutes until smooth. Reserve 3/4 cup batter; pour the remainder into pan.

Stir cocoa into the 3/4 cup reserved batter. Drop by spoonfuls over top of white batter. Using a small skewer or knife, swirl the cocoa batter into the white batter to incorporate it in a marble effect.

Bake in preheated oven for 30 to 35 minutes, until an inserted wooden pick comes out clean.