In a microwave-safe mug (at least 12 oz), whisk together the almond flour, coconut flour, granulated erythritol, cream of tartar, baking soda, cinnamon, and salt. The cream of tartar is what gives this that signature snickerdoodle tang, so don't skip it.
Add the egg, melted butter, heavy cream, and vanilla extract. Stir until the batter is completely smooth and well combined.
Microwave on high for 60 to 70 seconds, until the cake is puffed and just set on top. It should still be soft and pillowy in the center, not firm or rubbery.
While the cake cooks, mix together the softened butter, erythritol, and cinnamon in a small bowl until it forms a spreadable cinnamon butter.
As soon as the mug cake comes out, spread the cinnamon butter over the hot surface so it melts down into the cake, creating a warm, gooey, cinnamon-coated top. Serve immediately while everything is still warm and buttery.
