Preheat the oven to 180°C (350°F). Line an 8 x 8-inch baking pan with parchment paper.
Prepare the Crust: In a mixing bowl, combine almond flour, melted butter, and powdered sugar until well mixed. Press this mixture into the bottom of the prepared pan.
Bake the crust for 15 minutes, or until it turns golden. While it bakes, start on the filling.
Make the Lemon Filling: In another bowl, whisk together softened butter, lemon juice, and powdered sugar until it becomes glossy. Add in the eggs and egg yolk one at a time, mixing well after each addition. Stir in the coconut flour.
Assemble: Remove the crust from the oven, pour the lemon filling over it, and smooth the top.
Bake again for 20-22 minutes, or until the filling is set but slightly jiggly in the center.
Cool and Chill: Let it cool completely in the pan, then refrigerate for at least 2 hours before serving. Dust with additional powdered sugar if desired.