Mix the flour, cornstarch, and salt in a medium bowl. Set aside.
2 cups all-purpose flour, 2 teaspoons cornstarch, ½ teaspoon salt
In a large mixing bowl, beat the butter and granulated sugar for 2-3 minutes, until light and fluffy.
1 cup unsalted butter, ⅔ cup granulated sugar
Scrape the bottom and sides of the bowl, then add the egg yolk and vanilla extract. Beat for an additional minute.
1 large egg yolk, 1 teaspoon vanilla extract
Stir the flour mixture into the butter mixture on low speed until there is no visible flour remaining. Press the dough firmly into a ball.
Scoop 1 ½ tablespoon portions onto a parchment-lined tray. Roll the dough portions into balls then use your thumb or the back of a teaspoon to press an indention into each ball of dough. Press together any cracks, then refrigerate the dough for at least 1 hour.
When ready to bake, Preheat the oven to 350°F (177°C). Set the cookie dough 2 inches apart on a baking sheet lined with parchment paper or a silicone baking mat and bake for 6 minutes. Remove the tray from the oven and press the cookies down once again with the back of a spoon. Then return to the oven for an additional 6-8 minutes, or until the edges of the cookies just start to brown.
Cool on the baking sheet for 5 minutes, then transfer to a cooling rack to cool completely.
Prepare the icing by whisking 2 tablespoons of milk into the powdered sugar, along with the vanilla extract and corn syrup (if using). Once it is smooth, whisk in additional milk to reach your desired consistency.
Color the frosting to your liking. Then transfer into a disposable zip-top bag. Cut the corner off of the bag and squeeze the frosting into the center of each cookie. Allow an hour or two for the frosting to set.
Notes
Optional Ingredients:
The cornstarch will make the cookies softer. You can leave it out for a firm cookie.
Adding corn syrup to the frosting makes it glossy.
Storage:
Store cookies in an airtight container for up to 5 days.
Unfrosted cookies can be stored for 3 months.
Dough can also be frozen for up to 3 months. Thaw in the refrigerator before baking.
Tips:
Use a cool cookie sheet for each batch to help keep the dough from spreading too much.
Fill the cookies at least 2 hours before serving so that the icing has time to set.