In a microwave-safe mug (about 12 oz), whisk together the flour, sugar, baking powder, and salt.
Stir in the milk, melted butter, and vanilla extract until smooth. Don’t overmix—it’s okay if there’s a little lump or two.
Add optional mix-ins like chocolate chips or sprinkles if you’re feeling it.
Microwave on high for 60 to 90 seconds, until the cake is puffed up but still slightly gooey in the middle.
Let it sit for 1 minute—burned tongues are not part of the plan.
Enjoy straight from the mug. Or top with whipped cream, ice cream, or a drizzle of chocolate sauce.