Start by combining flour, baking powder, salt, sugar, cinnamon, and nutmeg in a mixing bowl. Cut the butter into cubes and work it into the dry ingredients until you get a crumbly texture. Add the egg and knead it all into a smooth dough. Flatten it into a disc, wrap it in plastic, and chill it for at least 30 minutes.

Mix flour, butter, sugar, cinnamon, nutmeg, hazelnuts, and salt. Use your hands to gently rub the mixture into fine crumbs. Chill the streusel topping in the fridge while you work on the custard and apples.

Heat milk and cream in a saucepan, keeping aside 5 tablespoons of milk. Mix the reserved milk with vanilla pudding powder or cornstarch and sugar until smooth. Once the milk is near boiling, add the pudding mixture and whisk continuously until it thickens. Stir in the egg yolks and continue cooking for a minute. Remove from heat, cover with plastic wrap to prevent a skin from forming, and set it aside to cool.

Preheat your oven to 356°F (180°C). Roll out the pastry dough to fit your springform pan, leaving a slight edge around the sides. Spread the cooled custard evenly over the base, followed by the chopped apples.

Sprinkle the chilled streusel evenly over the apple layer. Bake for 50–60 minutes, or until the streusel is golden brown and crisp. Allow the cake to cool completely before slicing.