Whisk flour, baking powder, and salt. Set aside.
2 Tablespoons (16g) all-purpose flour, ¼ teaspoon baking powder, ⅛ teaspoon salt
Whisk melted butter with sugar, then whisk in egg yolk and vanilla.
1 tablespoon (14g) unsalted butter, melted, 2 tablespoons (21g) granulated sugar, 1 egg yolk, ⅛ teaspoon vanilla extract
Stir in flour mixture and milk, then stir in rainbow sprinkles.
1 ½ tablespoons (3/4 ounce) milk, 1 teaspoon (5g) rainbow sprinkles
Spray a mug or ramekin (at least 7oz size) with nonstick cooking spray. Add the cake batter.
Cook for approximately 1 minute on high in the microwave, or until a toothpick comes out with just a few crumbs.
Cool before eating.