In a large bowl, whisk the flour, baking soda, cornstarch, and salt together. Set aside. 2 ¼ cups (280 g) all-purpose flour 1 teaspoon baking soda 2 teaspoons cornstarch ½ teaspoon salt
In a medium bowl, whisk the melted butter, brown sugar, and granulated sugar together until evenly combined. ¾ cup (1 ½ sticks, 170 g) unsalted butter ¾ cup (150 g) packed light brown sugar ½ cup (100 g) granulated sugar
Whisk in the egg and egg yolk until evenly mixed in. Whisk in the vanilla extract until evenly incorporated. 1 large egg 1 large egg yolk 1 Tablespoon pure vanilla extract
With a spatula or wooden spoon, stir in the dry ingredients and mix together. The dough will be soft and thick.
Cover the dough tightly and chill in the refrigerator for at least 2 hours (up to overnight). When ready to bake, take the dough out of the refrigerator and allow it to slightly soften at room temperature for 10 minutes.
Preheat oven to 350°F (180°C). Line large baking sheets with parchment paper or silicone baking mats. Set aside.
Using a small cookie scoop or use a measuring spoon and scoop dough into 2 heaping tablespoons portions (about 40 g).
Bake the cookies for 9-11 minutes until the edges are very lightly browned and the middle looks puffy but not wet. The cookies will continue to set as they cool. Cool on the baking sheet for 10 minutes. Then transfer cookies to a wire rack to cool completely.