Preheat oven to 175°C (350°F) and line a baking sheet with parchment paper.
In a bowl, mix almond flour, coconut flour, baking powder, cream of tartar, and salt.
In another bowl, cream butter and powdered erythritol until fluffy. Beat in egg and vanilla.
Gradually add dry ingredients to wet and mix into a soft dough. Chill for 20–30 minutes.
Mix granular erythritol and cinnamon in a small bowl.
Scoop tablespoon-sized portions of dough, roll into balls, and coat in the cinnamon mixture.
Place on baking sheet, flatten slightly, and bake 10–12 minutes until golden at the edges.
Cool on the sheet for 5 minutes, then transfer to a rack.