Butter a 13 x 9-inch baking dish and line with 2 sheets of parchment paper (one horizontally and one vertically), leaving a 1-inch over hang on all sides, set aside.
In a bowl of an electric stand mixer cream together butter, granulated sugar and vanilla extract until well combined. Slowly add in flour and cornstarch and mix until combined.
Press dough into an even layer in prepared pan, about 1/2 inch thickness (I did it with my hands first then used the bottom of a flat measuring cup to help smooth it out), and chill for about 30 minutes. Line 2 baking sheet pans with parchment paper. Set aside.
Lift dough from 13 x 9 baking dish using parchment overhang on all sides. Cut desired shapes using a cookie cutters and place on prepared pans, spacing them about 1 inch apart. Re-roll scraps as many times as needed to use up the dough.
Place cutouts in the refrigerator for at least one hour. Decorate with sprinkles and colored sugar if desired. When ready to bake, preheat oven to 350˚F.
Bake 8-10 minutes, or until edges are golden brown. Chill other sheet while one is baking. Cool completely then transfer to an airtight container and store at room temperature.