Preheat oven to 350 degrees. Line an 8 x 8 inch baking dish with parchment paper. Peel and dice apples and set aside.
In a large mixing bowl, whisk together flours, sea salt, nutmeg, cinnamon baking powder and vanilla powder (if using extract, mix with wet). In a small mixing bowl, whisk together maple syrup, eggs, coconut milk and extract. Mix then wet ingredients into the dry ingredients until evenly incorporated. Fold in apple pieces.
In a food processor, combine crumble toppings. Pulse until you reach the consistency of tiny pebbles. If you don't have a food processor, you can hand-chop the walnuts and mix everything together, you just won't have an even of a texture.
Empty 1/2 of your apple batter into the baking dish and smooth evenly with a spatula. Sprinkle half of the crumble on top. Add remaining batter and smooth, then remaining crumble.
Bake for 1 hour or until a toothpick comes out clean. Allow to rest for 30 minutes before removing a slicing.