Preheat oven to 350º.
In a stand mixer or with a hand mixer, cream your butter and sugar until light and fluffy.
Add in egg and vanilla extract and mix until fully combined, be sure to scrape the sides and incorporate that in as well.
Add in flour, baking powder, salt, and cornstarch and mix until a dough forms and no streaks of flour remain.
Take 3 tablespoons of dough and form into a ball. Press down lightly to create a thick disc shape.
Place 6 cookies to a tray (3 on the top row and 3 on the bottom), evenly spaced, as to avoid overcrowding as these cookies will spread as they bake. These yield 9 large cookies so you will need to do two batches in the oven.
Bake for 8 minutes or until the centers no longer look wet or doughy. They will remain pale so be sure to not overbake. The cookies will continue to bake on the tray even after being removed from the oven.
Remove from oven when done and let rest on the cookie tray for 5-10 minutes.
Transfer cookies to a cooling rack.
While the cookies are cooling, make the vanilla buttercream frosting.