Preheat oven to 350°F (175°C) and line two baking sheets with parchment. In a large bowl, whisk melted butter with both sugars until glossy. Whisk in egg and vanilla.

Sprinkle flour, baking soda, and salt over the wet mixture. Fold gently just until no dry streaks remain.

Fold in chocolate chunks until evenly distributed.

Drop heaping tablespoonfuls of dough 2 inches apart onto sheets. Bake 10 to 12 minutes, until edges are set but centers look soft.

Cool on sheets 5 minutes, then move to a wire rack. Sprinkle with flaky salt while warm and serve.