Preheat the oven to 375 degrees F and line baking sheets with parchment paper.
In a large bowl, add the butter and sugar.
Using an electric mixer, beat on medium-high speed until very pale and fluffy (about 3 to 5 minutes).
Add the egg and beat until well combined.
Add in the egg yolk and vanilla extract and beat until combined.
Add the flour, cornstarch, baking soda, and salt and beat on low until well combined.
Divide dough into four bowls and add food coloring. Mix well with a fork.
Chill dough for 30 minutes or more.
Make golf ball-sized dough balls using 2-3 colors of dough for each cookie.
Bake the cookies 8-12 minutes.
Remove from the oven and allow to cool on the cookie sheet for 2 minutes
Cool cookies on a baking rack.