In a medium stainless steel sauté pan set over medium heat, melt the butter. Occasionally swirl the pan as the butter melts. Continue cooking, allowing the butter to foam and make audible cracking and popping noises. Once the crackling quiets down, keep swirling the pan or stirring until the butter releases a nutty aroma and brown bits start to form at the bottom. When the bits turn amber, remove from heat and transfer the butter into a mixing bowl. Be sure to scrape the brown bits from the pan, as they carry a lot of flavor!

Add the granulated sugar and brown sugar to the hot butter and stir until well combined. Set aside and allow the mixture to cool to room temperature.

In a separate medium bowl, whisk together the flours, baking soda, baking powder, salt, and espresso powder (if using).

Once the butter mixture has cooled, whisk in the eggs, egg yolk, and vanilla until fully combined. Gradually stir in the flour mixture using a rubber spatula. Fold in the chopped chocolate and toffee bits. Wrap the dough in plastic wrap and refrigerate for at least 24 hours, but no more than 72 hours.

Allow the dough to sit at room temperature for about an hour until it’s soft enough to scoop.

Preheat the oven to 350ºF and line baking sheets with parchment paper.

Scoop the dough into 3-tablespoon portions using a large cookie scoop and place them onto the prepared baking sheets. The dough may be slightly difficult to scoop.

If you prefer, you can portion the dough onto a baking sheet and freeze until solid. Then, transfer the frozen dough balls into an airtight container and store them for up to 6 weeks.

Bake the cookies for 12 to 14 minutes, or until they are golden brown. Remove from the oven and, if desired, sprinkle flaky sea salt over the cookies. Let the cookies cool for 2 minutes before transferring them to wire racks to cool completely.

Store the cookies in an airtight container at room temperature for up to 3 days.