In a medium-sized pan, melt the butter over medium heat, swirling gently until it turns a rich golden brown and emits a nutty aroma.

In a mixing bowl, combine warm browned butter with granulated and dark brown sugar. Stir until smooth and cool to room temperature.

In another bowl, whisk together all-purpose flour, bread flour, baking soda, baking powder, fine sea salt, and optional espresso powder. Set aside.

Once cooled, whisk in the eggs and egg yolk, then mix in vanilla extract.

Fold the dry ingredients into the wet mixture until no dry flour remains.

Gently fold in chocolate chunks and toffee bits until evenly distributed.

Wrap dough tightly in plastic wrap and refrigerate for at least 24 hours.

Preheat oven to 350ºF and line baking sheets with parchment paper. Scoop chilled dough into balls on the sheets.

Bake for approximately 12-14 minutes until edges are golden brown and centers are soft.

Remove from oven and sprinkle with flaky sea salt while still warm. Cool on wire racks.