Preheat oven to 350°F. In the bowl of a stand mixer, use the paddle attachment to cream together the softened butter and sugars on medium-high speed until light and fluffy, about 3 minutes. Add egg and vanilla, and mix until combined, about 1 minute.

Add in the flour, cornstarch, baking soda and salt, and beat on medium-low speed until combined. (I break the rules and usually just sprinkle these dry ingredients evenly over the top of the dough and then mix them in. But if you’d like to make sure they’re super evenly mixed, just whisk together the dry ingredients in a separate bowl before adding them to the dough.)

Fold in the chocolate chips by hand until just combined.

Shape the dough into balls that are about 1-inch in diameter, and place them on a parchment-covered baking sheet. Chill in the refrigerator for at least 10-20 minutes, or until the dough is chilled through. (This will help prevent the cookies from spreading out too flat when they bake.)

Bake for 10-12 minutes until the edges are just set. The centers may look slightly undercooked. Cool cookies on the baking sheet for about 5 minutes, then transfer to a wire rack to finish cooling. Serve immediately or store in a sealed container for up to 1 week. You can also freeze the cookies for up to 3-4 months.