Make the lemon curd ahead and let chill. Full recipe is below.
Preheat oven to 350° and line a 9 inch square pan with parchment paper. Set aside.
Make the shortbread. In a medium bowl, combine almond flour, coconut flour, salt, vanilla, maple syrup, and butter. Stir together until it is well mixed and everything is incorporated evenly. Press ½ of the mixture into the bottom of the pan and bake for 10 minutes and then cool for 10 minutes.
Spread 1 cup of lemon curd evenly on the cooled crust and top with remaining shortbread. The best way to do this is to press the dough on a piece of wax paper into the size of the pan and then flip it on top of the lemon layer. Press any pieces together that come apart. Bake for 23-25 minutes, until golden brown around the edges. Let cool before frosting.
Make the frosting. In a small bowl, combine the powdered sugar and water and stir until smooth. Pour over the bars and place them in the fridge to set.
Store covered in the fridge for up to 10 days.