To start, preheat your oven to 350°F.
Grease a round, 9-inch baking pan, then dust with flour.
Put baking powder, salt, flour, sweetener, eggs, margarine, vanilla extract, potato milk, and mashed potatoes in a mixing bowl.
Slowly moisten the mixture by beating it.
Using an electric mixer, mix at medium speed for about 2 minutes or until smooth.
Pour the batter out into the baking pan, making sure to keep ¾ cup in the mixing bowl.
Add the cocoa powder to the ¾ cup of remaining batter and mix well.
You should now have two different batters; one white and one brown.
Use a spoon to scoop balls of the brown batter on top of the white batter.
With a knife or fork, gently swirl the two batters around to create the marble effect.
Bake between 30 and 35 minutes at 350°F or until a toothpick removes clean.