Grab a microwave-safe ceramic mug that holds at least one cup. A wider mug cooks more evenly than a tall, skinny one. Leave plenty of room at the top because the cake rises as it cooks.
Add the flour, sugar, baking powder, and a dash of salt to the mug. Stir them together with a small spoon until they look evenly blended. This is a great job for kids, because they love stirring.
Pour in the milk, melted butter, and vanilla extract. Stir until the batter is smooth, and scrape the bottom and corners of the mug. Dry flour hides down there, and it makes tough little pockets if you skip this.
Sprinkle in the teaspoon of sprinkles and give the batter one or two gentle stirs. Too much mixing can make the colors bleed into the batter. A light touch keeps the cake bright and cheerful.
Place the mug in the microwave and cook it for 70 to 90 seconds. The top should look just set but still a little shiny. Start with 70 seconds, since every microwave runs a bit differently.
Let the mug sit in the microwave for 1 full minute. The cake keeps cooking from the leftover heat and turns soft inside. If the center still looks wet, add 15 to 25 seconds, then rest it again.