To the bowl of a stand mixer fitted with the paddle attachment (or large bowl and electric hand mixer), cream together the unsalted butter, granulated sugar, and light brown sugar on medium-high speed until light and fluffy, about 5 minutes.
Stop, scrape down the sides of the bowl, and add the egg and vanilla extract. Beat until fully combined.
Add the cake flour, cornstarch, baking powder, baking soda, and a pinch of salt. Mix on low speed until just incorporated; don’t overmix.
Mix in 1 cup of sprinkles, and as desired, add the remaining ½ cup.
Using a medium 2-inch cookie scoop, form heaping 2 to 3-tablespoon mounds of dough. Place mounds on a large plate, flatten slightly, cover with plastic wrap, and refrigerate for at least 3 hours, or up to 5 days.
Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper or a Silpat mat.
Place mounds on the baking sheet, spaced at least 2 inches apart, and bake for about 8 to 9 minutes, or until edges have set and tops are just beginning to set.
Allow cookies to cool on the baking sheet for about 5 minutes before transferring to a wire rack to finish cooling.