In a bowl, whisk together all the dry ingredients: all-purpose flour, cornstarch, baking powder, and salt.
In another mixing bowl, cream together the butter, white sugar, and light brown sugar until light and fluffy, which takes about one minute. Make sure to scrape down the sides of the bowl.
Add the egg yolks and vanilla extract to the creamed mixture and mix until evenly combined. Scrape the sides of the bowl down again if needed.
Add the dry ingredients to the wet ingredients and mix until a dough forms.
Transfer the dough to a sheet of plastic wrap and wrap it tightly. Chill the dough in the fridge for an hour or in the freezer for 30 minutes.
After chilling, take about 2 tablespoons of dough and roll them into balls. Place them on a baking sheet, leaving some space between each, and press down in the center with your thumb to make a hole. You can also use a tablespoon measuring spoon to create the indentation.
Bake at 375°F for about 12 minutes or until the edges are slightly golden.
If the cookies have risen while baking, use the tablespoon measure to press down again in the center to flatten.
Allow the cookies to cool completely to room temperature, then fill and decorate them to your heart’s content!