Melt 1/5 cup (60 g) butter with the white chocolate. Beat the egg yolks with the sugar into foam. Add the chocolate paste to them and mix. Then add the milk, vanilla and sifted flour with the baking powder.
Finally, beat the egg whites to snow and mix them carefully with the remaining mixture. Pour the mixture into a large tray with a nonstick bottom, which you've pre-greased. Halve the apples, peel them and remove their seeds. Cut each half into thin slices but not all the way through, so that they still have their base.
Place a piece of butter on each apple piece. Bake in a preheated 356°F (180 °C) oven for about 45 minutes.