In a small bowl, mix together the softened butter, brown sugar substitute, and cinnamon for the swirl until smooth. Set aside.
In a large microwave-safe mug (at least 12 oz), whisk together the almond flour, coconut flour, granulated erythritol, baking powder, ground cinnamon, and salt until combined.
Add the egg, melted butter, heavy cream, and vanilla extract to the mug. Stir well until you have a smooth, lump-free batter.
Drop the cinnamon swirl mixture into the center of the batter in small spoonfuls. Use a toothpick or the tip of a spoon to gently swirl it through the batter in a circular motion, being careful not to fully mix it in.
Microwave on high for 70 to 80 seconds, or until the cake is set on top but still slightly soft in the center. Let it rest in the microwave for 30 seconds before removing.
While the cake rests, stir together the cream cheese, powdered erythritol, heavy cream, and vanilla extract until completely smooth and drizzle-able.
Drizzle the cream cheese glaze generously over the warm mug cake and serve immediately while the cinnamon swirl is still gooey.
