Brown the Butter: Start by melting the unsalted butter in a medium stainless steel sauté pan over medium heat. Swirl the pan occasionally to ensure even cooking until it becomes foamy and nutty aroma is detected.
Mix Sugars into Butter: Remove browned butter from heat and pour into a mixing bowl. Add both granulated sugar and dark brown sugar, stirring well to dissolve sugars into the warm butter.
Combine Dry Ingredients: In a separate bowl, whisk together all-purpose flour, bread flour, baking soda, baking powder, fine sea salt, and optional espresso powder until well combined.
Incorporate Wet Ingredients: Whisk in two large eggs and an extra egg yolk into the cooled butter-sugar mixture, followed by vanilla extract. Mix until combined and gradually fold in dry ingredients.
Add Chocolate and Toffee Bits: Stir in chopped semisweet chocolate and toffee bits until evenly distributed. Wrap dough tightly in plastic wrap and refrigerate for at least 24 hours.
Bake Your Cookies: Let dough sit at room temperature until soft enough to scoop. Preheat oven to 350ºF, line baking sheets with parchment paper, and scoop dough onto sheets. Bake for 12-14 minutes until golden brown.