Stir together the dry ingredients in a bowl - almond flour, coconut flour, baking powder, sweetener. Take care to break up any lumps.
In a second bowl, whisk the egg using a fork. Then add the melted and cooled butter plus the vanilla extract and whisk until combined.
Add the dry ingredients to the wet and stir until a smooth batter forms.
Grease a mug or ramekin with butter and fill in the batter. Microwave for 80 seconds @ 900W. (Baking times for different power levels are included in the notes below).
Enjoy straight from the mug or invert onto a plate.
