Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
Whisk together flour, baking powder, and salt in a medium bowl.
Cream the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add in the egg and vanilla extract; mix until well combined.
Gradually add dry ingredients to wet, mixing just until combined.
Fold in rainbow sprinkles by hand using a spatula.
Roll dough into balls (add more sprinkles if desired) and place on baking sheet.
Bake for 10–12 minutes, or until edges are lightly golden.
Cool on a wire rack before serving.