Whisk eggs and sugar until white and creamy.
Stir in oil and water briefly.
Fold in flour and baking powder.
Spread on a baking tray with baking paper.
Bake in preheated oven at 200°C for about 10 minutes.
Leave to cool.
Prepare a custard cream: Mix some apple juice with the custard powder and the sugar. Heat the rest of the apple juice in a pot, stir in the custard powder mixture and bring to boil while stirring.
Leave to cool a bit and fold in the diced apples.
Spread on the sponge.
Put in fridge to cool.
Whisk the cream until stiff and spread on the cold cake.
Lightly sprinkle with cinnamon.