Preheat the oven to 350°F (175°C).
In a bowl, mix together the almond flour, powdered erythritol, and melted butter until a crumbly mixture forms.
Press the mixture into the bottom of an 8-inch square baking dish.
Bake the crust for 10-12 minutes, until golden brown.
In another bowl, whisk together the eggs, granulated erythritol, lemon juice, and lemon zest.
Add the coconut flour, baking powder, and salt to the bowl and whisk until smooth.
Pour the filling over the hot crust and return the dish to the oven.
Bake for 18-20 minutes, until the filling is set.
Let the bars cool in the dish, then cut into squares and serve.