Add the melted and cooled butter, light brown sugar, granulated sugar, and vanilla to the bowl of a stand mixer. Mix on low speed until combined. With the mixer running on low, add the eggs one at a time, mixing after each addition.
Add the flour, cornstarch, baking soda, and salt. Mix just until the dry ingredients are incorporated. Do not over mix. Remove the bowl from the mixer. Use a rubber spatula to fold in the milk chocolate chips and confetti sprinkles until evenly distributed.
Line a baking sheet with parchment paper. Using a large cookie scoop, divide the dough into 24 evenly sized balls. Cover and refrigerate for 30 minutes or up to 24 hours.
Preheat the oven to 350°F. Arrange about 5 chilled dough balls on the prepared baking sheet, leaving approximately 2 inches between them. Keep the remaining dough refrigerated. Bake for 10–12 minutes, until the edges are lightly golden and just set. The centers should still look soft.
Immediately after removing the cookies from the oven, use a round cookie cutter or glass to gently shape them into even circles. If desired, press a few extra milk chocolate chips and sprinkles into the tops while they are warm. Let the cookies cool on the baking sheet for 5 minutes, then carefully transfer them to a wire rack to cool completely. Once cool, freeze the cookies for 15 minutes before assembling.