Preheat oven to 350 degrees F (177 degrees C).  Butter an 8 inch (20 cm) spring form pan and line the bottom of the pan with parchment paper.  Set aside.

For streusel topping:  In a large bowl, mix together the flour and ground cinnamon.  Cut in the butter with a pastry blender or fork until it resembles coarse crumbs.  Stir in the brown sugar and chopped hazelnuts.  Set aside while you make the cake batter.

For cake batter:  In a separate bowl whisk together the flour, baking powder, and salt.  Set aside.

In the bowl of your electric mixer, with the paddle attachment, cream the butter.  Add the sugar and beat until light and fluffy.  Add the egg and vanilla extract and beat until incorporated.  Add the flour mixture, alternately with the milk, and beat only until combined.  Spread the batter into the bottom of the prepared pan, smoothing the top with an offset spatula. 

Evenly arrange the apple slices on top of the cake batter and then sprinkle with the streusel topping.

Bake for about 45 - 50 minutes or until a toothpick inserted in the center of the cake comes out clean.  Remove from oven and place on a wire rack to cool slightly. 

Serve warm or at room temperature with vanilla ice cream or softly whipped cream.

Makes 8 - 10 servings.

Note:  To toast hazelnuts place in a 350 degree F (180 degree C) oven for 15 minutes or until fragrant and the skins begin to flake.  Remove from oven and place the hot nuts in a dish towel.  Roll up the towel and let the nuts sit (steam) for a few minutes then rub the nuts in the towel briskly to remove the skins.  Let cool before using.  Toasting the nuts not only removes the bitter skins but also enhances the sweet, rich, buttery flavor of the nut.