Preheat the oven to 180°C (350°F) with static/conventional heat.
Grease and flour the pan, tapping out the excess flour.
Peel the apples, remove the cores, and cut them into slices about 1 cm (0.4 in) thick. Arrange the slices in the pan.
Melt the butter.
Sift the flour into a large bowl and mix with the sugar, vanilla sugar, vanilla powder, and a pinch of salt.
Make a well in the center of the flour, crack the eggs into it, and mix from the center outwards to avoid lumps. Whisk thoroughly.
Add the warm melted butter and stir to combine.
Stir in half the milk, then add the rest, mixing well.
Note: You should have a smooth, lump-free batter. If desired, strain it through a fine sieve before pouring it over the apples.
Pour the batter over the apples and bake for about 50 minutes, depending on your oven. Start checking for doneness about 15 minutes before the end of the indicated time.
The clafoutis should be set and golden brown on top. Test for doneness by inserting a skewer into the center; it should come out clean.
Remove the clafoutis from the oven and transfer it to a wire rack. Serve warm or at room temperature. For a chilled version, let it cool completely before refrigerating.
Optionally, dust with a little powdered sugar before serving.