1. Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the long sides, and lightly grease the parchment and exposed pan edges.

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

3. Whisk in the eggs, one at a time, until just combined, then whisk in the vanilla extract. Avoid overmixing so the bars stay chewy, not cakey.

4. In a separate bowl, whisk together the all-purpose flour, baking soda, and fine sea salt.

5. Add the dry ingredients to the wet ingredients and gently fold with a spatula until no dry streaks of flour remain; the dough will be thick and soft.

6. Fold in the semisweet chocolate chips and milk chocolate chips (if using) until they are evenly distributed throughout the dough.

7. Scrape the dough into the prepared pan and press it into an even layer, smoothing the top with a spatula. Sprinkle flaky sea salt lightly over the surface if you like a sweet-salty finish.

8. Bake for 20–25 minutes, or until the edges are lightly golden and a toothpick inserted near the center comes out with a few moist crumbs but no wet batter.

9. Place the pan on a wire rack and let the cookie bars cool completely in the pan so they set properly.

10. Once cool, use the parchment overhang to lift the slab out of the pan, transfer to a cutting board, and slice into squares or rectangles. Serve and enjoy.