Brown the butter in a pan over medium heat until golden and nutty.
Pour into a bowl and let cool until warm, not hot.
Whisk in the brown sugar and granulated sugar.
Add egg, egg yolk, and vanilla. Whisk until smooth.
Add flour, baking soda, baking powder, and kosher salt. Mix until just combined.
Fold in chocolate.
Chill dough for at least 2 hours.
Scoop into 12 large dough balls, about 85g each.
Bake at 350°F for 13-14 minutes, until edges are set and centers look slightly underbaked.
Top with flaky sea salt immediately after baking.