Start by melting your butter in a saucepan over medium heat. Keep stirring as it melts—it’ll bubble, foam, and then slowly turn golden with little browned bits at the bottom. The smell? Nutty, rich, and absolutely heavenly. That’s your cue. Pull it off the heat and let it cool for about 10–15 minutes (patience here is key, otherwise you’ll end up cooking your eggs in the next step).
In stand mixer, mix together your cooled brown butter, brown sugar, and granulated sugar. Add in the vanilla, the whole egg, and that extra yolk (the secret to chewy cookies!). Stir until everything is smooth and glossy.
In a separate bowl, whisk your flour, baking soda, baking powder, salt and cornstarch. Slowly fold this dry mix into your wet ingredients—just until combined. Don’t overmix! Fold in your dark chocolate chunks and milk chocolate chips so they’re evenly spread through the dough.
Cover the bowl and pop it into the fridge for at least 2 hours. If you can wait overnight, even better.
When you’re ready to bake, preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Scoop 2–3 tablespoons of dough per cookie (go big here, they’re worth it!) and make sure to leave some space in between—they spread.
Bake for 10–12 minutes, just until the edges are golden brown but the centers still look a little soft. Pull them out, and while they’re still warm, sprinkle with flaky sea salt (if desired). Let them cool on the pan for 5 minutes before moving to a wire rack… if you can resist digging in right away.
