In a large bowl, mix together the almond flour, oat flour, baking powder, cinnamon and salt.
Peel the skin off the apples and cut them into quarters. Remove the centers and seeds of the apples and discard. Use the slicer side of a box grater or mandolin to cut the apples into thin slices. Slices are great because they don’t disrupt the structure of the cake and but they still hold up enough to shine through.
Add the apples to the the dry ingredients and mix everything together to coat the apples.
Place the eggs, applesauce, agave, and vanilla extract in a medium bowl. Whisk to combine. Pour the wet ingredients into the large bowl with the coated apples.
Use a rubber spatula to combined the ingredients.
Line an 8×8 square pan with parchment paper and transfer the cake batter to the pan. Spread it out in an even layer and set aside.
In a small bowl, use your hands to squeeze together the butter cubes, coconut sugar and cinnamon until they get clumpy and crumbly.
Sprinkle the crumbles over the top of the cake.
Bake for 35 minutes until the top is set and the center is no longer jiggly.
After baking, allow the cake to sit in the pan for about 15 minutes. This cake slices best when it’s room temperature or chilled so if you want super straight cuts, I suggest allowing it to cool. If you don’t care too much about the “perfect slice,” slice a knife around any edges touching the pan and then pull up the parchment paper to remove the cake from the pan. Cut into 12 slices.
In a small bowl whisk together the powdered sugar and milk. Drizzle over top the cake and enjoy! This cake would also be delicious alongside a scoop of vanilla ice cream of a cup of hot coffee.