Preheat oven to 325F (165C) and line a baking sheet with parchment paper. Set aside.

Combine butter and powdered sugar in the bowl of a stand mixer (or in a large mixing bowl and use an electric hand mixer) and beat on medium-high speed (use high speed if using a hand mixer) until lightened in color, fluffy, and creamy. This will take at least 5 minutes with a stand mixer and possibly a bit longer if using a hand mixer. If needed, pause occasionally to scrape the sides and bottom of the bowl to ensure all of the butter and sugar are being well creamed.

Add vanilla extract and salt and stir until combined.

In a separate bowl, whisk together flour and cornstarch.

Turn mixer speed to medium-low and gradually add flour, stirring until just combined.

Scoop dough into tablespoon-sized scoops and briefly roll between your palms to make smooth balls. Place on prepared baking sheet, spacing cookies 1-2 inches (2.5-5cm) apart.

Use a fork to gently, slightly flatten each cookie. If the cookie dough is sticking to the tines of the fork, dip the fork in a bit of powdered sugar first to prevent sticking.

Sprinkle cookies with nonpareils, jimmies, or sugar, if desired.

Transfer to 325F (165C) oven and bake for 12 minutes. Cookies will still be light in color when finished baking. Allow to cool completely on baking sheet before removing.