Position an oven rack in the center of the oven and preheat to 350 degrees F. Line a baking sheet with parchment.
Whisk together the butter, vanilla and eggs in a small bowl to combine.
Whisk together the almond and coconut flours, cocoa powder, salt, cream of tartar, cayenne, baking soda, 1/2 cup of the erythritol and 1 1/4 teaspoons of the cinnamon in a large bowl until combined. Using a handheld mixer, beat the egg mixture into the dry ingredients until fully incorporated. The dough will appear crumbly but will come together when molded by hand.
Using your hands, form the dough into heaping tablespoon balls and place them evenly on the prepared baking sheet; the cookies will not spread much while baking. Gently press down on each cookie with the palm of your hand.
Mix together the remaining 1/4 teaspoon erythritol and 1/4 teaspoon cinnamon and sprinkle on top of the cookies. Bake until firm and beginning to crack on top, 13 to 15 minutes. Let cool.
Erythritol, a sugar substitute, can be found in most health food stores or online.
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