Preheat the oven to 350°F/180°C. Line 2 baking sheets with parchment paper and set aside.
Pour the melted butter into a large mixing bowl. Then, whisk in the brown sugar and white granulated sugar until the mixture is homogenous.
½ cup unsalted butter, ½ cup brown sugar, ½ cup granulated sugar
Add in the large egg, egg yolk, and vanilla extract and whisk vigorously until well blended.
1 large egg, 1 egg yolk, 1 tablespoon vanilla extract
Add the all purpose flour, baking soda, and sea salt to the bowl. Use a rubber spatula to fold the dry ingredients into the wet ingredients. Stop mixing when no dry pockets of four remain and the cookie dough is just combined.
1 ½ cup all purpose flour, ½ teaspoon baking soda, 1 teaspoon sea salt
Use a medium cookie scoop to scoop even cookie balls and place them onto the lined baking sheets a few inches away from each other.
Bake each tray for 9-13 minutes. Do not overbake! The cookies are done when the edges appear set and light brown. The middles will be puffy and look slightly underbaked.
Remove the cookie sheets from the oven. Let the cookies cool for 5 minutes on the pan, then transfer them to cool for 10 minutes on a wire rack.
Sprinkle with flaky salt if desired and enjoy at room temperature!