In a medium-sized stainless steel pan, melt 1 cup of unsalted butter over medium heat, stirring constantly for about 5-7 minutes until golden brown.

In a mixing bowl, combine the warm browned butter with 1 cup of granulated sugar and ¾ cup of dark brown sugar. Whisk until smooth and glossy, allowing to cool for about 15 minutes.

In a separate bowl, whisk together 1 ½ cups of all-purpose flour, ½ cup of bread flour, 1 teaspoon of baking soda, 1 teaspoon of baking powder, 1 teaspoon of fine sea salt, and 1 teaspoon of optional instant espresso powder.

Once cool, add 2 large eggs and 1 additional yolk, followed by 2 teaspoons of vanilla extract. Whisk until combined.

Gradually fold the dry mixture into the wet ingredients using a spatula until just combined.

Gently fold in 10 ounces of chopped semisweet chocolate and 1 cup of toffee bits until evenly distributed.

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

Preheat oven to 350°F (175°C). Scoop dough onto lined baking sheets, spacing a few inches apart. Bake for 12-14 minutes until edges are golden.

Sprinkle warm cookies with flaky sea salt. Cool on baking sheets for a few minutes before transferring to wire racks.