Preheat oven to 350 degrees. Grease a 9x13 baking pan.
In a medium bowl, mix nuts, brown sugar, cinnamon, salt and butter until crumbly
In the bowl of a stand mixer, cream butter and sugar until light and fluffy. Add vanilla and mix well. Add eggs one at a time and beat until well incorporated.
In a medium bowl, mix flour, baking powder, baking soda, sea salt and cinnamon. Add half of the flour mixture to the butter mixture and mix on low. Add the creme fraiche and mix for about 15 seconds then add the remaining flour and mix until just incorporated.
Scatter half of the crumble topping in the bottom of the pan. Place batter in prepared pan and top with remaining crumble topping and blueberries. Bake for 35 minutes or until a cake tester comes out clean. Sprinkle with powdered sugar before serving.