Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper so the cookies release easily and the bottoms bake evenly.

Whisk the flour, cornstarch, baking soda, baking powder, and fine salt together in a medium bowl. Set the dry mixture aside while you prepare the buttery dough.

Beat the softened butter, granulated sugar, and packed light brown sugar for 2 to 3 minutes, until light, creamy, and slightly fluffy. Scrape down the sides of the bowl as needed.

Beat in the room-temperature egg and vanilla extract until fully combined. Add the dry ingredients and mix only until no dry flour remains; do not overmix.

Gently fold in the rainbow jimmies with a spatula. Work carefully to keep the sprinkle colors bright and prevent them from streaking through the dough.

Portion the dough into roughly 1 1/2- to 2-tablespoon balls. Arrange them on the prepared baking sheets with about 2 inches between each cookie, then add a few extra sprinkles on top if desired.

Chill the shaped dough for about 30 minutes. This step helps the cookies remain thick instead of spreading too quickly in the oven.

Bake each tray for 9 to 11 minutes, until the edges look set but the centers are still pale and slightly soft. Let the cookies cool on the baking sheet for 5 to 10 minutes before transferring them to a wire rack.