Preheat oven to 350 F. Line a 9x13 baking pan (8x8 works, too) with parchment paper to allow for easy removal after cooking/cooling.
Add all the ingredients for the shortbread crust to a food processor*. Process until well combined. *You can also mix by hand.
Press firmly into the lined pan and bake for 12-15 minutes until crust is just starting to golden.
While the crust cooks, use an electric or stand mixer to combine the lemon juice, lemon zest, eggs, maple syrup and vanilla extract. Add the coconut flour and arrowroot starch.
When the crust is done cooking, remove from the oven and pour the filling mixture over top. Do this immediately upon removing the crust – do not allow the crust to cool.
Lower the oven to 325F and put the pan back in the oven. Bake for 20 minutes or until set.
Remove from the oven and allow to cool completely before placing in the fridge.
Cool in the fridge for 2 hours to allow the lemon squares to fully set.
Remove from the pan and cut into squares. Sprinkle with raspberries or coconut, if desired. Serve.