Preheat oven to 350 degrees and line a greased 8×8 cake pan with parchment paper leaving an overhang on both sides.
Begin by making the Hazelnut Crumble topping. In a small bowl, combine the ingredients with a fork until well combined. Set aside.
In a large bowl, combine the dry ingredients. Add the almond flour, cinnamon, baking soda and salt. Whisk together to combine and remove any lumps.
In a medium bowl, combine the wet ingredients. Whisk together eggs, maple, melted butter, vanilla and lemon zest.
To the dry mixture, stir in 1/2 of the frozen wild blueberries, coating in the dry mixture, then add the wet ingredients. Combine with a spatula until well combined.
Transfer cake batter to the cake pan, smoothing the top. Top with remaining frozen wild blueberries, dispersing evenly all atop. Finish by sprinkling the Hazelnut Crumble topping all across the top.
Bake on the middle oven rack for 45-50 minutes, rotating the pan 180 degrees 2-3 times throughout cooking, until a toothpick inserted into the center comes out clean.
Cool cake in the pan for 10 minutes so cake will stay together.
Use parchment paper "handles" to delicately pull the cake out and transfer to a cutting board. Cut into 9 squares and serve warm.