Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat.
In a medium bowl, whisk together flour, baking soda, and salt. Set aside.
In another bowl, whisk melted butter with brown sugar and white sugar until smooth, about 30 seconds. Add egg yolk and vanilla, whisking until glossy and combined.
Pour wet ingredients into dry ingredients. Fold with a spoon or spatula until no dry flour remains. Fold in chocolate chips.
Cover the bowl and refrigerate the dough for 15 minutes (or up to 24 hours for deeper flavor).
Scoop dough into 6-8 balls (about 2 tablespoons/30 ml each) and place on prepared baking sheet, spacing 2-3 inches (5-7 cm) apart. Press extra chocolate chips on top if desired.
Bake for 10-12 minutes, until edges are golden brown and centers look slightly underdone and puffy.
Let cookies sit on the baking sheet for 5 minutes, then transfer to a wire rack. Sprinkle with flaky sea salt if using. Enjoy warm or at room temperature.