In a small bowl, combine the melted butter, milk, sugar, and vanilla.
Add the flour, baking powder, and salt. Mix just until the ingredient are combined.
Pour the batter into a large microwave safe mug.
Microwave on high for 90 seconds, then check for doneness (a toothpick inserted into the center should come out clean). If the cake is not fully cooked through, microwave for an additional 30 seconds, or until done.
Cool slightly then add the toppings of your choice and enjoy.