Peel, core and dice the apples, then set aside.
In a bowl, whisk together the eggs, salt, sugar and vanilla sugar until slightly thickened.
Add the Greek yogurt and vegetable oil, mixing until smooth.
Sift in the flour and baking powder, then mix gently until you have a smooth batter.
Fold the diced apples into the batter.
Grease a non‑stick skillet with the butter and heat gently.
Pour the batter into the skillet and level the surface.
Cover with a lid and cook on low heat for about 35 minutes, until mostly set.
Place parchment over the cake, flip it onto a plate, then slide it back into the skillet to cook the other side.
Cover again and cook on low heat for about 15 minutes, then cool slightly, slice and serve.