Heat the oven to 350°F (175°C) and line baking sheets with parchment paper. This temperature gives the cookies enough heat to set their edges while keeping the centers soft.

Whisk the flour, baking powder, baking soda, and fine salt together in a medium bowl. Mixing these ingredients first helps distribute the leaveners evenly through the dough.

Beat the softened butter and granulated sugar for about 2 minutes, until light and creamy. Add the egg and vanilla extract, then beat until the mixture is smooth and fully combined.

Add the dry ingredients gradually and mix only until no dry flour remains. Fold in the rainbow sprinkles gently so they stay intact and do not bleed excessively into the dough.

Cover and chill the dough for about 30 minutes. Chilling limits spreading and makes the dough easier to portion, especially when the butter is very soft or the kitchen is warm.

Scoop approximately 1 1/2 tablespoons of dough for each cookie and roll each portion into a ball. Place the balls about 2 inches apart on the prepared baking sheets, then press extra sprinkles onto the tops if desired.

Bake for 10–12 minutes per tray, removing the cookies when the edges are set and the centers still look slightly soft. The cookies should remain pale, so do not wait for them to turn brown.

Cool the cookies on the baking sheet for 5 minutes before transferring them to a wire rack. They will finish setting as they cool while retaining their soft, tender centers.