Brown the butter. In a medium saucepan over medium heat, melt the butter, swirling often, until it foams, then turns golden and smells nutty, 5–7 minutes. Pour into a large bowl, scraping in the browned bits, and let cool 10 minutes.

Mix the sugars. Whisk the brown sugar (200 g / 1 cup) and granulated sugar (67 g / ⅓ cup) into the cooled butter until smooth.

Add the egg and vanilla. Whisk in the egg, egg yolk, and vanilla (10 g / 2 tsp) until the mixture lightens slightly and looks glossy.

Add the dry ingredients. Sprinkle the flour (280 g / 2¼ cups), baking soda (5 g / 1 tsp), and salt (7 g / 1¼ tsp) over the wet ingredients and fold with a spatula just until no dry streaks remain.

Fold in the chocolate. Add the chocolate chips (340 g / 2 cups) and fold until evenly distributed.

Chill the dough. Cover and refrigerate at least 30 minutes — this firms the butter so the cookies hold their shape instead of spreading thin.

Scoop. Preheat the oven to 375°F. Scoop the dough into 3-tablespoon balls, spaced 3 inches apart on parchment-lined baking sheets.

Bake. Bake 11–13 minutes, until the edges are set and golden but the centers still look slightly underdone.

Finish and cool. Sprinkle with flaky salt immediately out of the oven, then let cool on the pan 10 minutes before moving to a rack.