Place all the ingredients in a microwave-safe mug and mix with a fork until smooth. Use the back of a spoon to smooth the top.
Microwave for 1 minute (you may need to adjust the cook time based on your microwave).
Carefully remove the mug from microwave (it will be hot), cover with a plate, and turn upside down to allow the biscuit to slide from the mug onto the plate.
Set the muffin on its die and cut into 4 even slices.