Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a mixing bowl, whisk together the almond flour, coconut flour, baking powder, salt, and half of the erythritol until evenly combined.

In another bowl, cream together the softened butter and the remaining erythritol until lighter in color and fluffy.

Add the egg, lemon juice, and lemon zest to the butter mixture and mix until smooth.

Gradually stir the dry ingredients into the wet mixture until a soft dough forms. If the dough is too wet, let it rest for 5 minutes.

Roll the dough into small balls (about 1 to 1.5 tablespoons each) and place them about 1.5–2 inches apart on the prepared baking sheet.

Flatten each ball slightly with the tines of a fork.

Bake for 10–12 minutes until edges are golden and tops are set. They will be soft when removed from the oven.

Let the cookies cool on the sheet for 5–7 minutes before transferring to a wire rack to cool completely.