In a mixing bowl, beat together softened butter and granulated sugar until the mixture is light and fluffy (around 3-5 minutes).
Add in the eggs and vanilla extract, mixing on low until well incorporated.
Combine flour, cornstarch, baking soda, and salt, then add to the wet ingredients, mixing on low until just combined.
Gently fold in ¾ cup of rainbow sprinkles using a spatula.
Chill the dough for at least 2 hours or up to 24 hours.
When ready to bake, let the cookie dough sit on the counter for 30 minutes and preheat the oven to 350°F.
Use a medium-sized cookie scoop (about 2 tablespoons of dough per cookie) to make 20 evenly sized cookies.
Roll the top of each cookie in rainbow sprinkles and place them about 2 inches apart on a cookie sheet.
Bake for 8-9 minutes or until the edges are just set.
Allow the cookies to cool for 3 minutes on the sheet before transferring them to a cookie rack to cool completely.