Spray a microwave-safe mug or ramekin with nonstick cooking spray.
Add the melted butter to the mug.
Whisk in the almond flour, egg, baking powder, and a pinch of salt until well combined.
Microwave on high for 90 seconds.
Let the bread cool slightly, then remove from the mug and slice.
Toast in a skillet or toaster for extra texture, if desired.