Melt the butter or coconut oil in a microwave-safe mug or ramekin, about 20 seconds.
Crack in the egg and whisk with a fork until fully combined with the melted butter.
Add the almond flour, coconut flour, baking powder, and salt. Stir until a smooth batter forms.
Microwave on high for 90 seconds until the bread is set and springs back lightly when touched.
Let it cool for a minute, then run a knife around the edges to release. Slice in half and toast if desired.