Cream cheese mixture: Combine margarine and cream cheese in a mixing bowl.
Beat until thoroughly blended.
Add sugar replacement and egg.
Beat until fluffy.
Blend in flour and vanilla.
Set aside.
Chocolate mixture: Melt chocolate and margarine in a small saucepan.
Stir to blend, and allow to cool.
Beat eggs on High in an electric mixer, until lemon-colored.
Beat in sugar replacement, fructose, and vanilla.
Gradually add flour and baking powder, beating well after each addition.
Blend in melted chocolate.
Spread half of the chocolate mixture on the bottom of a greased 9 in. (23 cm) square baking pan.
Pour cream cheese mixture over the surface of the chocolate mixture.
Cover with remaining chocolate mixture.
Take a knife and swirl through all layers.
Bake at 350°F (175°C) for 40 to 45 minutes, or until toothpick inserted in middle comes out clean.
Cool.