Grease a straight-sided microwavable cereal bowl or a 6" ramekin with melted butter
Mix together the remaining ingredients in a separate bowl with a fork until combined. The batter will be thick.
Pour muffin batter into greased bowl/ramekins, spreading it evenly.
Place bowl in the microwave, and cook, uncovered for 1 minute & 40 seconds, or until cooked through and puffed and the muffin has pulled away from the sides of the bowl.
Remove from microwave and release the muffin with a spatula. Using a serrated knife, cut the muffin in half lengthwise.
Optional: If you prefer your muffin pan toasted, heat a skillet over medium heat. Brush muffin halves with butter and pan fry for a minute, or until edges are nicely browned, or pop into the toaster to brown.