In a medium saucepan, add the unsalted butter and cook over medium heat. Swirl occasionally for about 5–7 minutes until it transforms to a beautiful golden brown with a nutty aroma and browned bits at the bottom. Remove from heat and carefully transfer the browned butter to a mixing bowl to cool.

Add granulated sugar and dark brown sugar to the warm browned butter, stirring well until smooth and fully combined, about 1–2 minutes. Allow this mixture to cool to room temperature.

In a separate large bowl, whisk together the all-purpose flour, bread flour, baking soda, baking powder, fine sea salt, and optional espresso powder.

Using a whisk, incorporate the eggs, egg yolk, and vanilla extract into the cooled butter mixture.

Gradually fold the dry ingredients into the wet mixture, mixing until just combined. Gently fold in the semisweet chocolate chunks and toffee bits.

Wrap the cookie dough tightly in plastic wrap and refrigerate for at least 24 hours, or up to 72 hours.

After chilling, allow the dough to sit at room temperature for about 1 hour to soften. Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.

Using a large cookie scoop or spoon, scoop out generous 3-tablespoon-sized portions of the dough and place them onto the prepared baking sheets, spacing them a few inches apart.

Bake the cookies in the preheated oven for 12–14 minutes or until they are golden brown around the edges but still soft in the center. Sprinkle with flaky sea salt if desired, then cool on wire racks.