Step 1: Brown the Butter - Melt the butter in a medium stainless steel sauté pan over medium heat. Swirl occasionally until golden brown and nutty, about 5-7 minutes.

Step 2: Mix Sugars - Pour the browned butter into a mixing bowl, stir in granulated and dark brown sugars with a whisk until combined. Cool at room temperature for 15-20 minutes.

Step 3: Combine Dry Ingredients - In a separate bowl, whisk together all-purpose flour, bread flour, baking soda, baking powder, sea salt, and espresso powder. Set aside.

Step 4: Mix Wet Ingredients - Add eggs, egg yolk, and vanilla to cooled butter-sugar mixture. Whisk until smooth, about 1-2 minutes.

Step 5: Combine Mixtures - Gradually add dry ingredients to wet, mixing until just combined. Fold in chopped chocolate and toffee bits.

Step 6: Chill Dough - Wrap tightly in plastic wrap and refrigerate for at least 24 hours, up to 72 hours for best flavor.

Step 7: Preheat Oven - About 30 minutes before baking, let dough sit at room temperature. Preheat oven to 350°F (175°C) and line baking sheets with parchment.

Step 8: Scoop & Freeze (optional) - Portion dough into 3-tablespoon balls, freeze until solid for future use in an airtight container.

Step 9: Bake - Bake dough balls on prepared sheets for 12-14 minutes until edges are golden. Reshape for uniformity immediately after baking.

Step 10: Finish & Cool - Sprinkle cookies with flaky sea salt. Cool for 2 minutes on the sheet, then transfer to a wire rack.