Preheat the oven to 375 degrees F and line baking sheets with parchment paper.
In a large bowl add the butter and sugar.
beat with an electric mixer on medium-high speed.
Beat 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 beat until combined. Add the flour, cornstarch, baking soda, and salt.
Beat on low until well combined.
Divide dough into 4 bowls and add food coloring, mixing 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
Melt a small handful of chocolate melts in a piping bag according to instructions on the bag.
Snip the tip off the piping bag and use it to add stitches to the cookies.
Follow the lines of different colors on the cookies.
Allow to dry for 30 minutes.