Preheat the oven to 350°F and grease a 9×9-inch baking pan with cooking spray.
Whisk together 1 cup almond flour with the coconut flour, lemon zest, and ¼ cup erythritol until well combined.
Cut in 3 tablespoons butter using a fork until it creates a crumbled mixture.
Press the dough into the prepared pan and bake for 15 minutes until golden then set aside.
Melt the remaining butter in a saucepan over medium heat.
Stir in the remaining erythritol along with the almond milk and lemon juice until the sweetener dissolves.
Slowly whisk in the eggs in a steady stream and cook on medium-low heat until it starts to thicken, about 5 to 10 minutes.
Pour the filling over the crust and bake for 15 minutes.
Combine the toasted coconut, the remaining 2 tablespoons almond flour, and the coconut oil in a blender and pulse into a crumbled mixture then sprinkle over the lemon squares.
Bake for 5 more minutes to toast the topping then cool before slicing to serve.