Place a 10-inch cast iron skillet into the oven. Set the oven to 220°C / 425°F and leave the pan there for a good 15-20 min.
In the meantime, prepare the batter. In a large bowl combine all the dry ingredients, one by one. Sift together: 7 tablespoons of plain flour, 3 tablespoons of buckwheat flour, followed by a teaspoon of baking powder. Give it a little whisk. Stir in the sugar (a tablespoon of brown cane sugar and about a teaspoon of vanilla sugar), a pinch of salt and whisk everything together.
Mix the liquid ingredients separately before adding to the dry mixture. Whisk 4 eggs, then pour in one cup of milk and keep whisking. It works better if the eggs and milk are used at room temperature. Whisk in a tablespoon of rum.
Now combine the two mixtures and whisk everything together until smooth. The batter should be a bit on the runny side.
Peel and thinly slice the apples. Carefully remove the hot pan from the oven and swirl in about 1-2 tablespoons of butter to coat. Pour in the batter. Spread the apple slices over and scatter with some roughly chopped hazelnuts. Put it back in the oven right away and bake for 20 minutes.
Dust the edges with some powdered sugar and drizzle a good quality honey on top.