Place the yogurt cheese in a medium-sized bowl, and sift the sugar over the cheese. Using an electric mixer, beat the cheese on medium-low speed until the mixture is smooth and creamy.
Add the lemon juice and vanilla extract to the cheese mixture, and continue to beat until well mixed and smooth. Set aside.
Slice the cake into 8 equal pieces, and place each slice on an individual dessert plate. Set aside.
If using a microwave oven, place the fudge topping in a glass bowl and microwave uncovered on high for 30 to 45 seconds, or until the topping is hot and spreadable, but not runny. If using a conventional stovetop, place the topping in a small saucepan and cook over low heat, stirring constantly, for 1 to 2 minutes, or until hot and spreadable.
Spread 1 tbsp of the hot fudge over each cake slice. Top with 1/4 cup of the cheese mixture, and serve immediately.