Preheat the oven to 350°F. Butter a 23cm (9in) loose-bottomed cake tin and then line with a circle of buttered parchment paper. To make the streusel, place the flour in a small bowl and rub in the butter, then stir in the brown sugar and hazelnuts.
To make the cake, beat the butter and caster sugar together with a hand held electric mixer in a bowl until light and fluffy. Beat in the eggs, a little at a time, adding one tablespoon of the flour when you have added about half of the beaten eggs. This will stop it from curdling. Sift over the rest of the flour and the baking powder and cinnamon, then mix lightly. Add the apple, hazelnuts and milk and fold in until thoroughly combined.
Spoon the cake mixture into the prepared cake tin and spread level. Scatter the streusel in an even layer on top of the cake mixture and bake for 50 minutes to 1 hour until the sides of the cake have shrunk slightly away from the tin and the streusel topping is golden brown. To test if the cake is done, insert a fine metal skewer into the middle. If it comes out clean, the cake is ready. Remove the cake from the oven and leave to cool in the tin for 15 minutes set on a wire rack.
To serve, cut the apple and hazelnut streusel cake into slices and arrange on plates with the warm custard.