Melt the butter in the pan, sweat the flour in it, gradually stir in the milk. Add the vanillin, salt and sugar, bring to the boil and allow to cool. Stir in vanilla yogurt.
Peel and chop the apples.
Preheat the oven to 200 degrees and grease the baking dish. Start with the vanilla sauce and alternately layer apples, lasagne sheets and sauce in the pan. Finish with plenty of sauce.