Preheat oven to 350F. Line a baking sheet with parchment paper.

In a medium bowl, whisk together almond flour, erythritol, baking powder, and salt until no lumps remain. Break up any sweetener clumps with your fingers.

Add softened butter, egg, lemon zest, lemon juice, and vanilla. Stir with a wooden spoon or spatula until a soft, cohesive dough forms. It will look slightly crumbly at first but keep mixing until it comes together.

Scoop dough into 1-inch balls (about 1 tablespoon each) and place 2 inches apart on the prepared sheet. Gently flatten each to about half-inch thick with your palm or the bottom of a glass. They won't spread much.

Bake until edges are just turning golden and tops look set, 11 to 13 minutes. The centers will still feel soft. Let cool on the pan for 10 minutes - they firm up as they cool - then transfer to a wire rack to cool completely.

Whisk powdered erythritol with lemon juice until smooth and pourable. Drizzle over cooled cookies with a spoon or dip the tops. Let glaze set for 15 minutes before serving.