In a medium bowl, whisk together the all-purpose flour, sifted cornstarch, powdered sugar and salt. Set aside.
In the bowl of a stand mixer, add the butter and vanilla extract. Beat until smooth. Next, add the egg yolk and mix just until incorporated.
Add all of the flour mixture and mix it on low speed, until combined, about 1 minute.
Scoop the dough out of the bowl and form it into a ball. Place it in the center of a sheet of plastic wrap and press it into about a 2-inch round. Wrap it tightly in plastic wrap and transfer to the fridge to chill for about 1 hour.
Preheat the oven to 350 degrees F.
When it’s done resting, in the fridge, transfer the dough to the center of a sheet of parchment. Roll it out slowly until it’s about 1/4-inch thick.
Using a 2.5-inch cookie cutter, stamp out the cookies, having them as close to each other as possible. With a spatula, transfer them to a parchment-lined baking sheet. Repeat until you work your way through all of the dough.
Score the tops of the cookies with a fork. Transfer to the oven to bake for about 11 to 13 minutes, until the cookies are a bit firm to the touch but have zero color on the edges. These cookies are baked just until set. Allow to cool on the baking sheets until room temperature.