Preheat your oven to 325°F (165°C). Line a large baking sheet with parchment paper.
In a medium bowl, whisk together the almond flour, erythritol, baking soda, and salt.
Add the egg, melted butter, lemon juice, lemon zest, and vanilla extract to the dry ingredients. Stir until a soft, slightly sticky dough forms. If the dough feels too soft, refrigerate it for 15 minutes.
Scoop tablespoon-sized portions of dough and roll them into balls. Place them on the prepared baking sheet about 2 inches apart and gently flatten each one to about half an inch thick with your palm.
Bake for 12 to 15 minutes, until the edges are just golden. The centers may look slightly soft but will firm up as they cool. Let the cookies cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely.
Make the glaze by whisking together the powdered erythritol and lemon juice until smooth. Stir in the lemon zest.
Drizzle the glaze over the cooled cookies with a spoon or fork. Let the glaze set for about 10 minutes before serving.