Brown the Butter: Melt 1 cup of butter in a stainless steel skillet or light-colored saucepan over medium heat. Swirl constantly as it foams and sizzles. After 4 to 6 minutes, dark golden specks will form on the bottom and release a rich, nutty hazelnut aroma. Immediately pour into a mixing bowl (scraping all the browned bits!) and cool for 15 minutes until lukewarm.

Toast Pecans: In a dry skillet over medium-low heat, toss chopped pecans for 3 to 4 minutes until fragrant. Set aside to cool.

Beat Wet Ingredients: Whisk cooled brown butter with brown sugar and granulated sugar for 2 minutes until glossy and thoroughly emulsified. Whisk in the whole egg, egg yolk, and vanilla extract until thick and pale.

Add Dry Ingredients: Fold in the flour, baking soda, cornstarch, and fine sea salt with a silicone spatula just until a soft dough forms with no dry streaks remaining.

Fold Mix-Ins: Fold in 1 cup of the toasted pecans and ¾ cup of the toffee bits (reserve remaining for topping). Cover and chill dough in the refrigerator for 30 minutes to firm up butter and deepen flavors.

Scoop & Top: Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper. Use a large 3-tablespoon cookie scoop to drop rounded mounds of dough 2 inches apart. Press reserved pecan chunks and toffee bits into the tops of each dough ball.

Bake to Golden: Bake for 10 to 12 minutes until edges are set and golden brown, but centers are still slightly soft and puffy. (Baker's Cue: Immediately swirl a round glass or cookie cutter around each hot cookie to nudge them into perfectly round bakery circles!).

Sprinkle Salt & Cool: Sprinkle with flaky sea salt while hot. Let cookies rest on the baking sheet for 8 minutes to firm up the molten toffee, then transfer to a wire rack to cool!