Preheat oven to 300℉. Line cookie sheet with parchment paper. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment (or use an electric hand mixer and large mixing bowl, cream the butter on high speed until it is smooth and light.
Gradually add the powdered sugar, beating well.
In a small bowl, whisk together the flour and cornstarch.
Gradually add the flour mixture to the creamed butter mixture on low speed, beating constantly until a stiff dough is formed.
Use your hands to roll the dough into balls of about 1 tablespoon each. If the dough is too sticky to handle, refrigerate for 30 minutes before rolling into balls.
Arrange the dough balls in a single layer on the prepared cookie sheet, leaving about 2 inches between each cookie. Using a flat bottomed glass that has been dipped in powdered sugar, press down gently on each dough ball to flatten. *Alternatively, press down gently using the tines of a fork, to create a ridged pattern in the cookie dough balls.
Bake in preheated 300 ℉. oven for 20 to 25 minutes, until just set.
Remove from the oven and allow the cookies to cool on the baking sheet for 10 minutes before transferring to a wire rack to cool completely.
Notes
*Store in an airtight container at room temperature for up to one week.Whipped shortbread cookies are not overly sweet, and have a nice buttery flavor and light texture.
Avoid over mixing the dough. Overmixing makes the dough sticky, and very difficult to work with. If the dough is mixed properly, you shouldn't need to chill it before baking.
For prettier whipped shortbread cookies, use the tines of a fork to create ridges in the dough. Or, feel free to use cookie stamps, or add mini chocolate chips or sprinkles to the dough.
If you prefer a slightly sweeter cookie, you can add a tiny bit more sugar, but just know this might affect the buttery consistency. A better idea is to make my No Chill Sugar Cookie Cut Out Recipe instead.