Preheat your oven to 350 degrees Fahrenheit.
In a large mixing bowl, combine softened butter, applesauce, eggs, white sugar, and vanilla extract until well blended.
Add almond flour, oat flour, baking powder, and salt into the wet mixture, stirring until everything combines into a smooth batter free of lumps.
Mix melted butter with oat flour, white sugar, and cinnamon until evenly combined for your filling; then create a crumble by combining slightly softened butter with brown sugar, almond flour, oat flour, and more cinnamon until crumbly.
Grease or line an 8×8 baking dish before pouring half of your cake batter into it; sprinkle half of your cinnamon sugar filling on top followed by swirls of remaining batter and finishing with leftover filling swirled in again.
Bake in preheated oven for 25 minutes before adding your crumble topping gently pressing it down; continue baking for another 18 minutes until fully cooked through before letting cool completely before slicing into squares.