Preheat the oven to 180C/350F. Grease a 9 x 11-inch pan with parchment paper and set aside.
In a small bowl, whisk together the almond flour, coconut flour, baking powder, and salt and set aside. In a mixing bowl, use a hand mixer to cream together the butter and brown sugar substitute until creamy. Add the eggs in, one at a time. Add the vanilla extract, then mix through the dry ingredients until combined.
In a small bowl, whisk together the crumb topping ingredients until combined.
Add half of the cake batter into the prepared baking pan, then spread half of the crumble mixture on top. Add the last half of the cake batter mixture to cover the middle layer crumble. Then, top with the remaining half of the crumble mixture.
Bake the coffee cake for 35-40 minutes, or until a skewer comes out mostly clean. Let the cake cool in the pan completely, before slicing and serving.
Notes
TO STORE: Leftovers can be stored in the refrigerator, covered, for up to one week. TO FREEZE: Place slices of the cake in an airtight container and store it in the freezer for up to 6 months.