Preheat oven to 350°F (180°C). Line baking sheets with parchment paper. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment or in a medium bowl using a handheld mixer, beat the butter and sugars on medium-high speed until creamed, about 2 minutes. Add the egg and vanilla and beat on high speed until well combined, about 1 minute.
In a small bowl, whisk together the flour, baking powder, baking soda, cornstarch, and salt. Add the dry ingredients to the wet ingredients and mix on low until just combined. Add the sprinkles and mix until evenly combined. The dough will be thick.
Using a large (3-tablespoon) cookie scoop*, drop the dough on a parchment paper-lined baking sheet.
Bake for 11-13 minutes until lightly browned on the edges. The centers will look very soft. Remove from the oven and allow to cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely.
* You can also use a 1½-tablespoon scoop which will yield approximately 16 smaller cookies. Check for doneness at the 10-minute mark.