Preheat oven to 350°F and line your baking sheets with parchment paper.
In a large mixing bowl cream your butter, sugar and brown sugar until light and fluffy.
Add your vanilla extract, egg, and egg yolk in stir until well combined.
Add your flour, cornstarch, baking soda, and salt. Stir until just combined.
Pour in about 3/4ths of your rainbow sprinkles (reserving the rest for the tops of your cookies) and stir in until evenly distributed.
Using a cookie scoop, scoop your cookie dough into 1-2 inch balls and place on your baking sheet lined with parchment paper.
Dip or top each cookie with more rainbow sprinkles.
Bake in the oven at 350 F for 8-10 minutes until edges start to brown.
Remove from oven, use a round glass or cookie cutter to scoot around the edges of the cookies to make them round and thicker. Let cool.
Serve, share & Enjoy!!