In a large bowl, cream together butter, egg, erythritol and vanilla extract until creamy. Beat in baking soda, cream of tartar, salt and almond flour until well combined. Dough will be stiff. Mix in coconut flour.Place dough in the fridge to chill for 1-2 hours.Preheat oven to 350 and line a baking sheet with parchment paper (or silicone baking mat)Mix cinnamon and erythritol together in a shallow dishTake dough out of the fridge and form dough into 2" balls. Place dough balls onto prepared baking sheet.Gently wet the bottom of a drinking glass and dip into the cinnamon mixture. Press the bottom of the glass onto a dough ball, flattening it slightly. Repeat until all dough balls have cinnamon coating.Bake for 13-15 minutes until the cookies have slightly browned. Remove from oven and let them cool and harden before removing from baking sheet.
Notes
Almond flour should be superfine, meaning without pieces of shells or hulls. Nutrition Facts as calculated in My Fitness Pal.