Add 14 tablespoons of melted and slightly cooled unsalted butter (it can be a bit warm, just not hot), 1 cup of brown sugar, and ½ cup of white sugar to a large mixing bowl. Cream together with a hand mixer or in a stand mixer with the paddle attachment until light and fluffy, about 4–5 minutes. Be sure to scrape the sides of the bowl a few times to make sure everything is fully incorporated.
Next, whisk in the egg, egg yolk, and 1 tablespoon of vanilla extract. Mix until fully combined, but don’t continue to mix. Just make sure it’s totally combined with no streaks of egg or egg yolk.
Sift in 2 cups of flour, 1 teaspoon of baking soda, and 1.5 teaspoons of kosher salt. Mix gently to incorporate using a rubber spatula. When the dough is almost all of the way mixed (there may still be streaks of flour), fold in most of the chocolate chips (reserve some for topping).
Scoop cookie dough using a large cookie scoop (#20 size) onto a parchment-lined baking sheet. Add some more chocolate and white chocolate chips to the top of the cookies. Cover with plastic wrap, transfer to the fridge, and chill the cookie dough balls for at least 30 minutes. You can also chill them for up to 2 days in the fridge or even freeze the dough balls (tightly wrapped) for up to 3 months if you plan to bake them later.
After the cookies have chilled and it's time to bake the cookies, line a large cookie sheet with parchment paper and preheat the oven to 350°F. Once the oven reaches 350°F (we recommend using an oven thermometer to be sure), it’s ready to bake your cookies! Place about 5–6 cookies on the baking sheet, leaving lots of room between them.
Bake for 13–15 minutes. The cookies are ready when the edges are golden, and the middle looks puffed, pale, slightly underdone, and a little gooey. The cookies will set up when they cool. Don’t overbake them.
Sprinkle the cookies immediately with flaky salt while they are hot. Allow them to rest on the baking sheet for 3-5 minutes so they can firm up enough to move them. You can also use a round biscuit cutter to “scoot” the edges of the cookies while they are still hot and give them a nice round shape. Once the cookies have firmed up enough to move, transfer them to a wire rack to cool completely. Add the remaining balls of cookie dough to the baking sheet and bake those while the first round is cooling.
Allow your baked cookies to cool and set up slightly before digging in, or else they will just fall apart. Once they have firmed up, enjoy!
Store leftovers in an airtight container at room temperature for up to 5 days.