1. Preheat the oven to 350ºF and line a greased 9×9 inch baking dish with parchment paper. Lightly grease the parchment. (see photos below)

To make the shortbread crust:

1. In the bowl of your stand mixer or large bowl if using hand beaters: beat the egg, melted butter, and sugar until well combined. Add the almond flour, arrowroot flour, coconut flour, and sea salt and mix on low until mostly combined and then turn it up to medium-high until it forms a dough.
2. Press the dough into the bottom of the prepared pan. Bake for 12-14 minutes, or until it is dry to the touch.
3. Wipe out the bowl you were using.

To make the filling:

1. In the same large bowl, beat the cream cheese, egg, arrowroot flour, vanilla and sugar on low until it is mostly combined, and then turn up to high and beat until it is completely smooth. Scrape down the sides as needed.
2. Pour onto the par-baked crust and smooth out. An offset spatula comes in handy here. 
3. Sprinkle on the blueberries, and then the topping.
4. Return the baking dish to the oven, and bake for 350ºF for 30 minutes, or until the topping is golden brown and the center is just set. 
5. Allow to cool completely before cutting. Place on a rack at room temperature for about an hour before transferring to the fridge to finish cooling. 

To make the topping:

1. Combine the almond flour, butter, and sugar in a small bowl and stir to combine. It should be the consistency of wet sand.