Crumble

1. In a medium bowl, combine the butter, both flours, sugar, and salt. Using your hands or a pastry cutter, combine the ingredients together until everything is well combined and moistened. Allow a few small bits of butter to remain. Cover and refrigerate until needed.
2. To make the streusel. In a food processor, combine the pecans, sugar, cinnamon, and nutmeg and pulse until the pecans are ground, but not becoming a paste. Set aside.

Cake

1. Position a rack in the middle of the oven. Preheat to 350°F.
2. Line and grease a 10-inch round cake pan or spring form pan - Use butter to lightly grease the pan and use parchment paper to line the pan for the best results.
3. With a hand mixer or in a stand mixer fitted with a paddle attachment, cream the butter, sugar, and salt on medium-high speed until light and fluffy and the sugar begins to dissolve into the butter.
4. Incorporate the vanilla and eggs, one at a time, beating well after each addition. Add the flours, hazelnut meal, baking powder, and baking soda.
5. Add in the crème fraiche and mix until incorporated.
6. Scoop half of the batter into the prepared pan and smooth it with a small offset spatula.
7. Sprinkle the streusel evenly over the top of the batter.
8. Top with the rest of the batter, coaxing it into covering the streusel layer. Top with the crumble.
9. Bake for 1 hour and 10 minutes, or until fully baked through. Allow to cool in the pan on a wire rack for about 15 minutes.
10. Place a flat plate on top of the cake and pan. Carefully invert the cake onto the plate by flipping both upside down. Then lift the pan off the cake.
11. Gently pull the parchment paper away from the cake. Rest the serving plate on the bottom of the cake and turn the cake right-side up onto the plate.

Icing

1. In a small bowl, combine the confectioners' sugar, heavy cream, and vanilla. Stir with a fork to combine.
2. Drizzle over the top of the cake before serving.