In the bowl of a stand mixer with a paddle attachment, or with a hand mixer, beat the butter until it’s soft, smooth, and light.
Scrape down the bowl, add the sugars to the beaten butter, and mix until they are fully incorporated and lighter in color.
Beat in salt followed by the egg.
Mix until the batter looks uniform.
In a medium bowl, sift together flour, cornstarch, and baking soda.
In three additions, beat this slowly into the butter-egg mixture until you see a few dry streaks remaining.
Switch to a stiff spatula and gently mix the cookie dough until no more dry flour remains.
Thoroughly whisk together the cinnamon-sugar mixture.
Form balls of dough with a medium-sized cookie scoop or a tablespoon measure and plop them in cinnamon-sugar, swirling to coat.
On a parchment-lined cookie sheet, place the dough balls 3 inches apart.
Flatten the dough slightly (to about 1/2 - to 3/4 -inch thick) using the bottom of a glass, and sprinkle a little more cinnamon sugar over the flattened surface.
Put into the preheated oven.
Bake for 7 minutes, rotate the pan, and bake for another 5-7 minutes (12-14 minutes total), until the edges of the cookies look golden and the tops are crackled but still pale.
Cool on the baking sheet and eat as soon as possible.
Snickerdoodles will keep in an airtight container, at room temperature for 3 days.