Step 1

Preheat the oven to 375°F. Line two baking sheets with parchment paper. Set aside.

Step 2

In the bowl of a stand mixer fitted with the paddle attachment, mix the cold butter, brown sugar and granulated sugar on low speed. Gradually increase the speed and cream the butter and sugars together for 4 to 5 minutes, until pale and fluffy. There should be no visible chunks of butter in the mixture. Scrape down the sides of the bowl.

Step 3

Add the egg and egg yolk, one at time, stopping to scrape down the sides of the bowl between each addition.

Step 4

Mix in the vanilla.

Step 5

In a separate bowl, whisk together the flour, baking soda, cornstarch and salt.

Step 6

Add the dry ingredients to the butter and egg mixture in two additions. Mix on low speed until just combined. Do not over mix.

Step 7

Add the chocolate chips all at once and mix for just 10 seconds to combine. Do not over mix the dough.

Step 8

Remove the bowl from the mixer and use a large cookie scoop to divide the dough into 2 ounce cookie dough balls. Place on a parchment lined baking sheet.

Step 9

Roll the cookie dough balls between the palms of your hands until smooth. Work quickly so that the dough does not get too warm.

Step 10

Place the balls of cookie dough 2 to 3 inches apart on the prepared cookie sheets. Top each ball with a few extra chocolate chips if desired. Bake in the center rack of the oven for 9 to 10 minutes. Check the cookies around 7 minutes into baking. If the cookies are still mounded on top, lift the cookie sheet about 3 to 4 inches above the baking rack and drop it firmly. Repeat if needed. This will flatten out the cookies and help create delicious pools of melted chocolate.

Step 11

The cookies are done when the edges are set and slightly golden brown. The centers of the cookies will still look pale and slightly under baked. Do not overbake the cookies or they will be dry and crispy instead of soft and chewy.

Step 12

Remove the cookies from the oven and allow to cool on the baking sheet for 5 to 10 minutes before carefully transferring to a cooling rack.

Step 13

Sprinkle with flakey sea salt before serving if desired.