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.