One-Bowl, No-Mixer, No-Chill, Chocolate Chip Cookie For One

egg yolk from a large egg (discard white or save for another use)

unsalted butter, very soft

granulated sugar

light brown sugar, packed

vanilla extract

plus 2 tablespoons all-purpose flour

baking soda

cornstarch

kosher salt, or to taste

semi-sweet chocolate chips, plus more for adding on top before and after baking if desired (recommended)