Choose the right mug: Use a microwave-safe mug that holds at least 12 ounces.
Lightly grease it with butter or nonstick spray.
Mix dry ingredients: In the mug, whisk together almond flour, coconut flour, sweetener, baking powder, cinnamon, and a small pinch of salt until no lumps remain.
Add wet ingredients: Add the egg, melted butter, almond milk, and vanilla. Stir until the batter is smooth and evenly combined.
Make the cinnamon swirl: In a small bowl, mix melted butter, cinnamon, and brown-style sweetener. Drizzle over the batter and use a toothpick or knife to gently swirl it in.
Microwave: Microwave on high for 60–90 seconds.
Start checking at 60 seconds. The cake should look set on top but still moist. Avoid overcooking.
Make the glaze: Stir powdered sweetener, vanilla, and just enough milk or cream to reach a thick drizzle.
Add a tiny pinch of salt if desired.
Finish and serve: Let the cake rest for 1 minute to set. Drizzle with glaze and enjoy warm.