Preheat your oven to 325°F (165°C). An adequately heated oven is crucial for achieving that perfect cookie texture.

In a large mixing bowl, take your softened unsalted butter and white granulated sugar. Using a hand mixer or stand mixer, cream them together until the mixture is light and fluffy. This process typically takes about 3-5 minutes. Don’t rush this; it helps incorporate air into the batter, creating a light and airy cookie.

Gradually sift in the plain all-purpose flour, cornstarch, and fine kosher salt into your creamed butter mixture. The goal is to combine everything gently, so you don’t lose that airiness you just built in the previous step. Use a spatula to fold the flour into the mix until just combined. You might see a bit of flour left; that's perfectly alright—don't over-mix!

At this point, if you want to add the pure vanilla extract, pour it in and fold gently again. You'll start smelling that wonderful scent!

Now you have a cohesive dough. Turn it out onto a floured surface. Shape the dough into a rectangle about 1/2 inch thick. You can use your hands to pat it down or a rolling pin to create an even surface.

Using a sharp knife or a cookie cutter, cut the dough into squares or rectangles. Place the cut pieces onto a baking sheet lined with parchment paper. Leave a bit of space between each cookie for expansion.

Before popping your shortbread in the oven, sprinkle a bit of sparkling sugar generously on each piece. This will create a delightful crust.

Bake in your preheated oven for 20-25 minutes. You’re looking for a light golden hue around the edges. Keep an eye because every oven varies.

Remove from the oven and let the shortbread cool on the baking sheet for about 10 minutes. After that, transfer them to a wire rack to cool completely. They’ll firm up further as they cool.