1. In a medium saucepan, melt the butter over medium heat and cook until it foams and browns, with a nutty aroma. Remove from heat and let cool for 10 minutes.

2. Add brown sugar and granulated sugar to the brown butter and whisk to combine. Add the egg, egg yolk, and vanilla extract. Whisk until smooth and glossy.

3. In a separate bowl, mix the flour, baking soda, and salt. Gradually stir the dry mixture into the wet ingredients until combined.

4. Fold in toffee bits and chopped chocolate evenly throughout the dough.

5. Chill the dough for at least 1 hour or overnight for best results.

6. Preheat oven to 350°F (175°C) and line baking sheets with parchment paper. Scoop 2-tablespoon portions of dough and place spaced apart on the sheet.

7. Bake for 10–12 minutes, until edges are golden and centers are just set.

8. Sprinkle flaky sea salt on top immediately after baking. Let cool on the baking sheet for 5 minutes, then transfer to a wire rack.