Begin by preheating the oven to 200°C (392°F) using both upper and lower heat (Convection bake function: 180°C (356°F)). Line the baking sheet with parchment paper.
Combine plain flour (all-purpose flour), cornstarch (cornflour), and baking powder. Sift the mixture and set it aside.
In a large empty bowl, add sugar and eggs. Use a hand mixer or a similar tool to beat the eggs for about 8 minutes. This step is crucial in creating a light and airy sponge cake because it increases the volume as the sugar-egg mixture incorporates air. Do not skip or shorten this step.
Now, either using a spatula or the hand mixer at the lowest speed, gently fold in the flour mixture. Do not overbeat the batter when using a hand mixer. You want to retain the air in the egg-sugar mixture to achieve a light and airy sponge cake.
Transfer the batter to the prepared baking sheet, smooth it with a spoon or spatula, and bake for 12 to 15 minutes or until it turns slightly brown. Keep a close eye on your sponge cake to prevent it from burning.
Take the sponge cake out of the oven and let it cool down completely.
To make the filling , wash the fresh fruits, cut off the stems (if available), and pat them dry with paper towels. Then, cut them into small pieces and place the sliced fruits in a bowl. Mix them and set them aside.
Soak the gelatin for about 5 minutes. Meanwhile, whip the chilled cream with the vanilla sugar until it becomes stiff.
In a separate bowl, combine mascarpone, yogurt, sugar, and vanilla extract. Mix until well combined. Then, gently fold in the whipped cream.
Squeeze out any excess water from the gelatin leaves. Place the squeezed gelatin leaves into a small saucepan and heat them over low heat until they become liquid. Add a portion of the yogurt-mascarpone cream to the liquid gelatin and stir it well.
Now, fold the gelatin mixture into the remaining yogurt cream. Then, add the previously set aside cut fruits.
Next, spread the fruity mascarpone cream evenly on the completely cooled sponge cake and smooth it out. Then, refrigerate the cake for at least an hour. Shortly before the end of the chilling time, prepare the Tortenguss.
To make the cake glaze, cook the Tortenguss packets with sugar and orange juice according to the package instructions.* If you're using cornstarch or cornflour, pour about two-thirds of the orange juice into a medium pot and bring it to a boil. In the meantime, add the remaining orange juice, cornstarch, and sugar to an empty cup or a small bowl, and combine them well. Slowly pour the cornstarch mixture into the pot with the boiling orange juice while stirring continuously. Let it cook for about half a minute, stirring constantly. Immediately after cooking, spread the Tortenguss over the cream using a tablespoon.
Let the cake cool in the fridge for 1 hour, and then enjoy. These cream bars are perfect for serving a large group of people.