Cream together butter and sugar on medium high for about 2 minutes. Scrape down the bowl as needed.
Add the egg, salt, and vanilla (+ any other flavorings) and mix again on low just until combined.
Add the cornstarch and about half of the flour. Turn the mixer on, then slowly add the remaining second half of the flour until all mixed in. It's important to measure your flour properly. I suggest weighing it, but if you don't have a food scale, be sure to spoon and level your flour and NOT pack it. Divide the dough into 2 and place each one in between two pieces of parchment or wax paper. Roll it out until the dough is evenly about 1/4 - 1/2 inch thick, depending on your preference. (I prefer a slightly thicker cookie as they stay soft longer.) And yes, by rolling it between parchment it means you don't have to add any extra flour!
Chill for about 20 minutes in the freezer or 1 hour in the fridge, or overnight. Dough will last in the fridge for a few days. I keep it between the pieces of parchment paper as I chill to keep the dough fresh.
While chilling, preheat your oven to 375° F. Once dough is chilled, remove the top piece of parchment paper and begin cutting! Re-roll remaining dough and cut.
Bake for 9-12 minutes, depending on your oven. Watch the cookie closely towards the end so they don't over brown. You know they're ready when the tops are not wet anymore and the bottoms are cooked. Ever so slightly beginning to golden on the bottom is fine, but a fully golden brown bottom is too long.
Let them sit on the pan for a few minutes after baking, then transfer to a cooling rack. Store airtight for a few days or freeze for up to a few months!