Preheat oven to 400 degree F. Cut 2 pieces of parchment paper and using a ruler draw 12-2 ½ inch lines spaced about 3 inches apart. Place the papers written side down onto the baking sheets and set aside.
Whisk together the flour, cornstarch, baking soda, and salt.
1 ½ cups flour, 1 tbsp cornstarch, ¾ tsp baking soda, ½ tsp salt
In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using a hand mixer, beat butter, powdered sugar, and granulated sugar until light and fluffy, about 3-5 minutes. Add the egg and vanilla and beat until combined.
½ cup butter, ½ cup powdered sugar, ⅓ cup granulated sugar, 1 egg, 1 tsp vanilla extract
Mix in the dry ingredients until there are no more dry spots. Finally, beat in the milk.
1 tbsp milk
Transfer the batter to a large piping bag fitted with a ½ inch round tip. Pipe the batter onto the lined baking sheets using the guides you marked earlier.
Bake in preheated oven for 10-12 minutes. Remove from oven and let cool 5 minutes before transferring to a cooling rack. Repeat with remaining batter.