It was posted on a forum without attribution - I've found some variations of it, but will just list it here.

melt 1 tablespoon butter in a mug (I use a 10 oz. ramekin - to get the shape I want)

the dry ingredients are:
pinch of salt
1/2 tsp. baking powder
3 tablespoons almond flour (I use superfine)

and 1 egg.

I melt the butter, mix all the dry ingredients - mix the egg into the dry, then mix that batter well into the sl. cooled melted butter.

Microwave for 90 seconds. Let cool a bit, then split in half and pan toast in butter.