In a medium bowl whisk the flour, cornstarch, salt, and set aside.
Use an electric mixer to cream the softened butter and sugar until light and fluffy. Add the room temperature egg and vanilla extract and mix until smooth. Scrape down the sides of the bowl with a rubber spatula as needed. Add the dry ingredients and mix until evenly combined. The dough will be very thick.
Preheat your oven to 350°F and line a large rimmed baking sheet with parchment paper or a silicone baking mat. (Do not grease)
Form the dough into a large ball and divide it in half or quarters (optional, makes rolling out easier). Lightly flour a clean surface and your rolling pin and roll the dough out to about ¼” thick.
Cut out the cookie shapes and arrange them on the prepared pan. Gather up the dough scraps, re-roll, and repeat until all of the dough is used. Sprinkle the dough and your rolling pin with more flour as needed to prevent the dough from sticking.
Bake in the preheated oven for 10-12 minutes or until just starting to lightly brown on the bottom. The cookies will set and harden as they cool. Allow to cool for a few minutes on the pan and then transfer to a plate or a cooling rack to cool completely.
Make the icing: In a large bowl whisk the powdered sugar, light corn syrup, milk, clear vanilla extract, and salt until smooth. Decorate the cookies as desired once they've completely cooled. If adding sprinkles or candy make sure you add them before the icing sets.