Preheat the oven to 350 degrees F.
Grease an 8 in x 8 in pan.
In a large bowl, mix 1/2 cup almond flour, 3/4 cup chestnut flour, 1/4 tsp ground nutmeg, 1 tsp ground cinnamon, and 1 tsp baking soda.
In another bowl, mix 1 tsp vanilla, 1 tsp apple cider vinegar, 1/4 cup maple syrup, and 3 medium eggs until the mixture thickens. Add 2 Tbs of melted coconut oil and mix again. Then pour the dry ingredients into the bowl of wet ingredients and mix. Pour the batter into a greased 8 in x 8 in pan and spread the batter out evenly with a rubber spatula.
In another bowl, mix together the crumb topping. Add 1/2 cup almond flour, 1 tsp ground cinnamon, and1 tsp unsweetened cocoa powder to a bowl, mix. Add 2 Tbs maple syrup, and 1/4 cup crushed chestnuts to the bowl and mix. Add 1 tsp melted coconut oil and mix. If you have the right crumble consistency, stop here. If not, add another tsp. (I only used one)
Pour the crumbles on top of the batter in the pan, making sure to evenly spread them out.
Bake everything for 25 minutes, until the edges of the cake look crisp and you can poke a knife into the middle of it and it comes out clean.
Serve with a bit of maple syrup sprinkled on top.