1. Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang for easy lifting, and lightly grease it.

2. In a large mixing bowl whisk together the melted butter, brown sugar, and granulated sugar until the mixture looks thick, glossy, and slightly cooled.

3. Whisk in the eggs, egg yolk, and vanilla extract until the batter is smooth and slightly lighter in color.

4. Sprinkle the flour, baking soda, and salt over the wet ingredients and gently fold with a spatula just until no streaks of dry flour remain.

5. Fold 1 1/2 cups of the chocolate chips into the batter, then scrape the thick dough into the prepared pan and spread it into an even layer.

6. Scatter the remaining 1/2 cup of chocolate chips over the top, lightly pressing them into the surface so they stick.

7. Bake for 23–27 minutes, or until the edges are golden and a toothpick inserted in the center comes out mostly clean with a few moist crumbs.

8. Cool the pan completely on a wire rack, then use the parchment overhang to lift the slab out and cut into 20–24 squares.