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

In a medium bowl, stir together almond flour, coconut flour, baking soda, salt, and erythritol until evenly distributed.

In a separate bowl, cream the softened butter with lemon juice, lemon zest, vanilla extract, and egg until glossy and somewhat loose.

Pour the wet mixture into the dry and stir until a cohesive dough forms. If too wet, add almond flour gradually until it holds shape.

Scoop rounded tablespoons of dough onto the prepared baking sheet, spacing them about 2 inches apart. Bake for 10–12 minutes until edges are lightly golden and centers are set.

Let cool on the baking sheet for 3–5 minutes, then transfer to a wire rack to cool completely.