First, make the cream: in a saucepan, mix the egg with the stevia sweetener (or sugar), the vanilla sugar, the cornstarch and the milk using a whisk.
Bring the mixture to a boil while whisking until the cream thickens.
Cover the surface of the cream with cling film and let it cool to 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 frothy.
Incorporate the milk and oil, mix, sift and add the flour with the baking powder and mix
Finally add the apples cut into thin slices
Pour half of the preparation into a rectangular mold (27×27 cm) lined with baking paper
Add a few spoons of cream then cover with the rest of the mixture
Using a piping bag, pipe the pastry cream in a checkerboard pattern
Finally, bake the cake at 170°C for 40-45 minutes
Sprinkle with icing sugar and serve
