Thumbprint Cookies

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, about one minute. Scrape down the sides of the bowl.
Then add the egg yolks and vanilla extract and mix until combined evenly. Scrape the sides of the bowl down again as necessary.
Then add the dry ingredients to the wet ingredients and mix until it starts to combine and a dough forms.
Transfer the dough and wrap it tightly with Saran Wrap and then chill it in the fridge for an hour or in the freezer for 30 minutes.
After chilling them, take about 2 tablespoons of dough and roll them into balls. Place them on a baking sheet, spaced apart slightly, and press down in the center with your thumb to make a hole in the center. You can also use a tablespoon measuring spoon to make the indentation in the center.
Pop in the oven at 375°F to bake for about 12 minutes or until just a bit golden on the edges.
If they have risen a bit while in the oven, just take the tablespoon measure and press down again in the center to flatten it back down.
Allow them to cool completely to room temperature and then fill and decorate as you’d like!