In a small pan or microwave, allow the butter to melt into the milk and let it cool. Meanwhile, preheat the oven to 180°.
In a large bowl, hand whisk the eggs and sugar, then combine the milk, flour and baking powder. Mix well until it turns into a thick batter.
Peel the apples and slice them very thinly with a mandolin slicer or knife.b
Pour the apple slices into the previously prepared batter and gently mix everything with your hands, so that each apple slice is well-covered with batter.
Transfer the apples to a 20 cm diameter cake pan or into a muffin pan (for several muffin-sized cakes), and try to level them as much as possible to get a flat surface. Cook at 180°C for approximately 35 minutes. If you decide to use disposable pans, choose aluminum ones, because paper ones do not allow the cake to bake well, and the dough will be too moist.
When the cake is ready, let it cool down and dust it with confectioner's sugar just before serving.