Put butter in a wide microwave-safe mug (or small microwave-safe cereal bowl).
Microwave for 10 seconds or until melted.
Add almond flour and baking powder to melted butter and mix until combined.
Add the egg white and mix until combined.
Put the mug back in a microwave and microwave on High for 90 seconds.
Remove the keto bread from the mug and slice in half (like English muffin).
If desired, put the keto bread in the toaster and toast it.