Preheat oven to 350. Spray a 9 inch round cake pan with cooking spray and then line with parchment paper.
Spread the 2 tablespoon softened butter on the bottom of the cake pan. Sprinkle with the sweetener and top with the chopped chocolate.
Add all the cake batter ingredients to a medium bowl and mix with a wooden spoon until smooth. Drop scoops onto the chocolate topping. The batter will be very thick. Gently smooth to cover the chocolate.
Bake for 25 min until the cake is slightly firm to the touch. Cool for 15 minutes. Run a knife around the edge and then invert onto a serving platter. Peel off the parchment paper. Serve immediately.