In a microwave proof mug, combine all ingredients. Mix well with a fork. If adding berries or chocolate chips to your batter, do so now, but don't stir. They will fall automatically as it cooks.NOTE: If you're only making one mug pancake, crack your egg in a small container, beat, and then pour half of the egg into the mug and save the rest of your egg for another pancake later.
Cook in the microwave on high for 2 minutes.
Remove from microwave and cut an X in the top of the pancake. Slip your pat of butter inside to melt.
Top with syrup or your topping of choice. Serve immediately!
If you'd like to make a smaller version, swap the 1/3 cup measurements for 1/4 cup and leave everything else the same!