In your favorite mug (at least 12 oz size), mix together the flour, sugar, baking powder, and a dash of salt.
To the same mug, add the melted butter, milk, and vanilla extract. Stir until the batter is smooth. Make sure to reach the bottom to avoid any flour pockets.
Fold in the rainbow sprinkles if you're in the mood for a little extra joy.
Cook on high in the microwave for about 70-90 seconds. The top should look set when it's done. Let it cool for a minute before digging in. Be careful, it's hot!