Preheat oven to 300 degrees F. Prepare two baking sheets by lining them with parchment paper. Set aside.
In the bowl of your stand mixer fitted with the paddle attachment, beat butter and powdered sugar on low for about 30 seconds.
Increase the speed to medium and continue to beat until light and fluffy, about 3-4 minutes.
Stir in the flour, cornstarch, salt, and vanilla just until combined.
Roll cookie dough into 1 ½ inch balls and place on the prepared baking sheets, leaving an inch or two between each cookie.
Using a fork, lightly press down on top of each ball of dough to flatten it to about a ½ inch thick.
Sprinkle nonpareils on top of each cookie.
Bake for about 20-22 minutes, or until the bottoms are lightly golden brown.
Video
Notes
Butter: If you'd like to use salted butter, omit the kosher salt. Also, cool room temperature means the butter is about 68F. The butter should dent when pressed but the stick shouldn't lose its shape. Butter that's too warm can cause the cookies to bake out.
Flour: It's important to either weigh the flour or sift or stir it to break it up, lightly spoon it into the measuring cup, and level. Avoid packing the flour so that you don't end up with crumbly cookies.
Kosher salt: Or 1/4 teaspoon of table salt.
Vanilla extract: You can change up the flavor of the cookies by using different types of flavorings, like almond extract, in place of par or all of the vanilla extract.