Prepare the cream:
In a bowl, beat the mascarpone with lemon zest, lemon juice, powdered sugar, and whipping stabilizer until creamy.
Gently fold in the whipped cream until smooth and fluffy.
Chill the cream in the fridge until firm.
Make the lemon batter:
In a large bowl, beat the eggs with lemon zest, sugar, vanilla sugar, and salt using a hand mixer or stand mixer until light and fluffy.
Slowly stir in the sparkling water and lemon juice until well combined.
Mix dry ingredients:
In a separate bowl, mix flour, cornstarch, and baking powder.
Gradually fold the flour mixture into the egg mixture, alternating with the cooled melted butter. This keeps the batter airy and smooth.
Bake the batter:
Place a baking frame on a baking tray lined with parchment paper and divide the batter into two halves.
Spread the first half evenly in the frame and bake at 180°C (top and bottom heat) for about 7 minutes until golden and cooked through.
Remove the sponge from the oven, carefully peel off the parchment paper, and let cool on a wire rack.
Repeat with the second half of the batter.
Assemble the lemon bars:
Once the sponge cake has cooled, spread the cream evenly on one cake layer.
Cut the second cake layer into even bars and spread the remaining mascarpone cream on top.
Let the bars chill in the fridge for at least one hour to firm up and for the flavors to meld.