Preheat the oven: Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

Make the dough: In a large mixing bowl, beat the softened butter and powdered erythritol together until smooth and creamy. Add the vanilla extract and mix well. Gradually add the almond flour, coconut flour, and salt to the butter mixture, stirring until a dough forms. The dough should be soft but not sticky.

Shape the cookies: Roll the dough into 1-inch balls and place them on the prepared baking sheet. Gently press each ball down with your palm or a fork to flatten into round cookie shapes, about 1/4 inch thick.

Bake the cookies: Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden. Be careful not to overbake, as shortbread cookies can dry out quickly.

Cool and serve: Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely. If you want, drizzle or dip the cookies in melted sugar-free chocolate for an extra treat.