Lightly spray a microwave-safe mug with cooking spray. (I like to use an 8 oz coffee mug but any large mug will do.) This will make clean up easier.
Add the flour, baking powder and sugar to the mug and use a fork to whisk together these dry ingredients.
Add the liquid ingredients: melted butter, milk, and vanilla. Whisk together all the ingredients with a fork until most of the lumps are gone. The pancake batter may just have a few small lumps. Use a small spatula to scrape down the sides of the mug to make sure you get all the pockets of dry ingredients clinging to the inside of the mug. You may want to put paper towels under the mug as you cook it, as it may drip over the sides a little.
Microwave pancake for 1 minute, 20 seconds on medium heat. (Individual results depends on the wattage of your microwave oven - if the pancake is not cooked, try another 10 seconds.)
Garnish! Top off you pancake with a slab of butter, syrup and fresh berries and enjoy your fluffy pancake!