Preheat the oven and mix the dry ingredients

• Place racks in the upper and lower thirds of the oven and preheat to 375 °C.
• Whisk the flour, salt and baking soda in a small bowl and set aside.

Brown the butter

• Cook one stick of butter in a large saucepan over medium heat, swirling often and scraping the bottom with a spatula, until it foams and browns.
• Scrape the browned butter and brown bits into a large heatproof bowl and let cool 1 minute.
• Cut the remaining butter into small pieces and add to the brown butter so it melts without foaming.

Whisk in the sugars, eggs and vanilla

• Add the dark brown sugar and granulated sugar and whisk, breaking up clumps, until no lumps remain.
• Add the egg and egg yolks and whisk until the sugar dissolves and the mixture is smooth.
• Whisk in the vanilla extract.

Fold in the dry ingredients and chocolate

• Fold the reserved dry ingredients into the butter mixture with a spatula just until no dry spots remain; do not overmix.
• Coarsely chop the chocolate and fold it in.
• If the dough slumps when scooped, stir it and let it rest until the scoops hold their shape.

Portion the dough

• Using a scoop, portion out 16 balls of dough.
• Divide between two parchment-lined rimmed baking sheets.

Bake the cookies

• Bake until deep golden brown and firm around the edges, rotating the sheets if the cookies brown very unevenly.
• Let cool on the baking sheets.
• Sprinkle with flaky sea salt if desired.