In a small bowl, mix the egg with a fork or a whisk.
Stir in the almond flour, salt, and baking powder.
Drizzle in the melted butter and stir until mixed.
Spray a mug with non-stick spray (or grease with butter); pour in the batter.
Microwave for 90 seconds or until cooked.
If you want to slice this keto bread, I recommend waiting a couple of minute for it to cool first; it will be easier to cut.
I cool my keto mug bread right in the mug for a minute or two, slide it out of the mug onto a plate, and let it cool for another minute.