Preheat the oven to 200C/180C Fan/Gas 6. Grease the inside of an ovenproof dish or deep baking tray (approximately 27x20cm/10¾x8in).
Sieve the flour and baking powder into a large bowl.
Separate the eggs, placing the whites in a large bowl and the yolks in a jug. Add the vanilla, cinnamon and buttermilk to the jug and briefly whisk to combine.
Add the egg yolk mixture to the flour mixture and whisk to form a thick, lump-free batter – do not over-mix.
Sprinkle the caster sugar into the egg whites and whisk to soft peaks. Add a couple of spoonfuls of the whisked whites to the thick batter and gently stir through to combine.
Add the remaining whisked whites to the batter mixture and use a metal spoon to gently fold in, trying to retain as much air as possible. Fold the apple cubes through the batter until just combined.
Carefully spoon the pancake mixture into the prepared baking dish or tray. Lay the apple slices on top of the batter and sprinkle over half of the hazelnuts, followed by the demerara sugar.
Bake for 18–20 minutes until the pancake has risen and the top is golden.
Remove the pancake from the oven and sprinkle with the icing sugar to decorate. Cut into squares and serve with crème fraîche, maple syrup and the remaining hazelnuts.
