Start by melting 1/2 cup of unsalted butter in a microwave-safe bowl. Heat it in 15-second intervals until it's melted, then set aside. I like to let my butter sit for about 10 minutes once it's melted to let it cool down a bit. This makes the dough easier to scoop and helps the cookies spread less while they bake.
While the butter cools, preheat your oven to 350°F/175°C and line two large baking sheets with silicone mats or parchment paper.
Whisk together 1 1/3 cups flour, 1 tsp cornstarch, 1/2 tsp baking soda, and 1/2 tsp salt in a large bowl. Set aside.
Pour the melted and cooled butter, 2/3 cup packed brown sugar, and 1/4 cup granulated sugar into a large bowl. Whisk the ingredients together by hand for about a minute, until the mixture is smooth and no lumps remain. It should become lighter in color as it's whisked.
Whisk in 1 large egg and 1 tsp vanilla extract. Stir until the egg is fully incorporated.
Pour the dry ingredients into the butter/sugar mixture and stir together with a wooden spoon or spatula. Mix until no visible streaks of flour remain. The dough will be soft/sticky, but that's how it's supposed to be! Mix in 1 cup of chocolate chips or chunks of your choice.
Use a spoon or cookie scoop to shape 1 1/2 Tbsp cookie dough balls and place them 2-3 inches apart on the prepared baking sheets. They need a lot of space because they spread quite a bit while they bake!
Bake both trays of cookies together for 11-13 minutes, and rotate the pans halfway through to help them bake evenly. The cookies are ready when the edges just start to brown, but the centers still look slightly under-baked. The cookies will continue to bake on the sheet and firm up once removed from the oven, so it's always best to err on the side of under-baked!
Place the cookie sheet on a wire rack and let the cookies cool fully on the pan. I like to press a few extra chocolate chips on top of each cookie while they're still warm and add a garnish of flaky sea salt.
Enjoy warm with a glass of milk, or store fully cooled cookies in an airtight container at room temperature for up to 3 days.