Brown the Butter: Melt unsalted butter in a medium saucepan over medium heat. Swirl frequently as it foams and sizzles. After 4 to 5 minutes, golden foam turns to nutty amber liquid filled with speckled brown milk solids. Pour into a heatproof bowl and let cool for 20 minutes until lukewarm.

Toast the Pecans: Add chopped pecans to a dry skillet over medium heat. Toast for 3 to 4 minutes until deeply fragrant and lightly golden. Remove from heat and cool.

Mix Wet Foundation: Whisk the cooled brown butter, dark brown sugar, and granulated sugar vigorously for 2 minutes until glossy and combined. Whisk in eggs one at a time, followed by pure vanilla extract, until batter is thick, ribbon-like, and pale.

Incorporate Dry Ingredients: Whisk flour, baking soda, and fine sea salt in a bowl. Fold into the wet ingredients using a silicone spatula just until a soft dough forms. Fold in toasted pecans and chopped dark chocolate chunks.

Chill & Portion: Cover dough and refrigerate for 30 minutes (this hydrates the flour and concentrates toffee notes). Preheat oven to 350°F (175°C) and line two large baking sheets with parchment paper. Scoop 3-tablespoon mounds of dough (approx. 55-60g) spaced 2.5 inches apart.

Bake & Finish: Bake for 10 to 12 minutes until edges are set and golden-brown, while centers remain soft and puffed. Immediately out of the oven, use a round cookie cutter or glass to gently "scoot" cookies in a circular motion for perfect round edges. Sprinkle generously with flaky Maldon sea salt. Cool on the pan for 10 minutes, then transfer to a wire rack.