Preheat the oven to 350°F. Line a baking sheet with parchment paper.
Measure flour, baking soda, cornstarch, and salt into a bowl and set aside.
Beat butter in the bowl of a mixer until light and fluffy.
Add sugars and mix until light and fluffy (1-2 minutes)
Add egg and vanilla and mix until thoroughly combined.
Slowly beat in flour mixture, adding milk just before everything is completely combined.
Fill the bottom corner of a piping bag with your batter.
Cut the tip of your bag so that you have a ½-inch wide tip.
Pipe the cookies, leaving 3 inches in between cookies, and making each cookie roughly 2 ½ inches long. These cookies will expand significantly.
Bake for 8-11 minutes.
Let cool on a cooling rack.