To start, mix together the dry ingredients (flour, sugar, baking powder, and salt) in a microwave-safe mug. I used an 8-ounce clear glass mug.
In a small bowl mix the wet ingredients(milk, vanilla, and butter) and add to dry ingredients and mix until the cake batter is smooth.
Microwave the mug cake for 1-2 minutes, depending on your microwave's wattage. The top of the cake should be set and springy to the touch. My microwave is a 750 watt and it took 2 minutes.
Let the mug cake cool for about 5 minutes, then top with your favorite toppings.
Notes
The key to a fluffy texture is to mix the wet and dry ingredients separately before combining them. This helps to ensure that the batter is evenly mixed and doesn't become too dense.
Using room temperature milk also helps to create a lighter cake, as cold milk can cause the melted butter to solidify.