2. First, make the cream: in a saucepan, mix the egg with the stevia sweetener (or sugar), vanilla sugar, cornstarch, and milk using a whisk.
Bring the mixture to a boil while whisking until the cream thickens.
Cover the surface of the cream with plastic wrap and let it cool at room temperature.
In a bowl, beat the eggs with the stevia sweetener (or sugar), salt, and vanilla. Mix well for 3-4 minutes with an electric mixer until the mixture becomes frothy.
Incorporate the milk and oil, mix, sift, and incorporate the flour with the baking powder and mix.
Finally, add the thinly sliced apples.
Pour half of the mixture into a rectangular mold (27x27 cm) lined with parchment paper.
Add a few spoonfuls of cream, then cover with the remaining mixture.
Using a pastry bag, pipe the pastry cream in a checkerboard pattern.
Finally, bake the cake at 170°C for 40-45 minutes.
Sprinkle with powdered sugar and serve.