Preheat oven to 350°F (180°C)
Place salted butter in light-colored saucepan over medium heat
Melt butter completely, then continue cooking while stirring frequently until deep golden color and nutty aroma form
Immediately transfer brown butter to a heat-safe bowl
dissolve salt and baking soda in brown sugar mixture
cream together brown butter, light brown sugar, and granulated sugar
Add egg, milk, and vanilla extract to butter, mixing until fully emulsified
Sift all-purpose flour into the wet mixture and fold in
toss chopped dark chocolate and chocolate chips into dough
Chill dough for at least 30 minutes up to overnight
Portion dough into 24 balls and place on baking sheets
Sprinkle flaky sea salt on top
Bake until lightly golden and edges are crisp (4-5 minutes per side)