Preheat the oven to 325F and grease a 10-inch oven proof skillet (cast iron is best).
In a large bowl, whisk together the almond flour, sweetener, cream of tartar, baking soda, cinnamon, and salt. Stir in the butter, egg, and vanilla extract until well combined.
Spread the dough in the prepared skillet and smooth the top. Bake 22 to 25 minutes, until the cookie is set around the edges and just barely set in the center. Remove and let cool 10 minutes before serving.
In a small bowl, whisk together the sweetener and cinnamon. Sprinkle over the cookie. To serve, scoop the cookie out by the spoonful or cut into slices when it's completely cool.
Top with low carb vanilla ice cream or lightly sweetened whipped cream.