Prepare the dough: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. In a large mixing bowl, combine the almond flour, coconut flour, sugar-free sweetener, salt, and baking powder. Whisk the dry ingredients together thoroughly to ensure even distribution and avoid clumps.

Mix wet ingredients: In a separate bowl, beat the softened butter until smooth and creamy. Add the egg, lemon zest, lemon juice, and vanilla extract, then mix well until the mixture is fully combined and slightly fluffy.

Combine ingredients: Gradually add the dry ingredients into the wet mixture. Stir until a soft dough forms. If the dough feels too sticky, let it rest for a few minutes to allow the coconut flour to absorb moisture. If it feels too dry, add a small splash of lemon juice or a teaspoon of water.

Shape the cookies: Scoop out small portions of dough and roll them into balls using your hands. Place them onto the prepared baking sheet, spacing them about 2 inches apart. Gently flatten each ball with your fingers or the back of a spoon to form cookie shapes.

Bake the cookies: Transfer the baking sheet to the preheated oven and bake for 12–15 minutes, or until the edges turn lightly golden. Keep an eye on them to avoid overbaking, as almond flour cookies can brown quickly.

Cool and serve: Remove the cookies from the oven and let them cool on the baking sheet for about 5 minutes before transferring them to a wire rack. Allow them to cool completely, as they will firm up as they rest. Optionally, dust with extra sugar-free sweetener before serving for a light finishing touch.