Heat unsalted butter in a cast iron skillet over medium heat and cook 5-6 minutes until golden and nutty, stirring often so the milk solids brown evenly. The butter should smell toasted and look amber with browned specks; turn off heat and cool 15 minutes.

Whisk the cooled browned butter with brown sugar, packed and granulated sugar until glossy and the sugar looks dissolved. Whisk in the large egg plus 1 egg yolk, then whisk in vanilla extract until smooth.

Stir in all-purpose flour, baking soda, and salt until just combined. Fold in dark chocolate chips and toffee bits so they’re evenly distributed without overmixing.

Cover the dough and chill for 1 hour in the refrigerator. It should firm up enough to scoop without spreading too fast.

Preheat the oven to 350F and line sheet pan(s) with parchment. Scoop 2-tablespoon balls onto the parchment-lined sheets, spacing them apart for spreading.

Bake 10-12 minutes at 350F until golden at the edges but still slightly soft in the centers. For a visual cue, the tops will look set and the edges will be deeper brown.

Sprinkle the cookies with flaky sea salt while warm. Cool 5 minutes on the sheet pan so the cookies finish setting before moving them.