Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper to prevent sticking.
In a large mixing bowl, use an electric mixer to cream together the softened unsalted butter, granulated sugar, and brown sugar until the mixture is light and fluffy, about 2-3 minutes.
Beat in the large egg and vanilla extract to the butter-sugar mixture until well combined.
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and sea salt.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined.
Using a spatula, gently fold in the rainbow sprinkles until they are evenly distributed throughout the cookie dough.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto the prepared baking sheet, leaving space between each cookie.
Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden.
Remove the cookies from the oven and let them cool on the baking sheet for 5 minutes before transferring them to a cooling rack to cool completely.
