Step 1: Brown the Butter - In a medium sauté pan over medium heat, melt 2 sticks of unsalted butter. Swirl the pan frequently as it foams and begins to turn a rich golden-brown color, releasing a nutty aroma—this takes about 5–7 minutes. Once browned, carefully pour the butter into a mixing bowl and allow it to cool to room temperature.

Step 2: Mix Sugars - In the bowl with the cooled browned butter, add ½ cup of granulated sugar and 1 cup of dark brown sugar. Stir vigorously until the sugars are fully incorporated into the butter, creating a smooth mixture.

Step 3: Combine Dry Ingredients - In a separate bowl, whisk together 1 ½ cups of all-purpose flour, 1 cup of bread flour, 1 teaspoon of baking soda, ½ teaspoon of baking powder, 1 teaspoon of fine sea salt, and 1 teaspoon of optional instant espresso powder.

Step 4: Add Eggs and Vanilla - Gently whisk 2 large eggs and 1 egg yolk into the sugar-butter mixture until fully combined. Then, add in 2 teaspoons of vanilla extract.

Step 5: Incorporate Chocolate and Toffee - Stir in 10 ounces of chopped semisweet chocolate and 1 cup of homemade or store-bought toffee bits.

Step 6: Chill Dough - Wrap your cookie dough in plastic wrap, ensuring it's well-sealed, and refrigerate for at least 24 hours—up to 72 hours enhances the flavor even more.

Step 7: Prep for Baking - When ready to bake, remove the dough from the refrigerator and let it sit at room temperature for about an hour to soften slightly. Preheat your oven to 350ºF and prepare two lined baking sheets.

Step 8: Bake - Place the cookie sheets in the preheated oven and bake for 12–14 minutes, or until the edges are golden brown and the centers appear slightly underbaked.

Step 9: Cool and Season - Immediately after removing the cookies from the oven, sprinkle with flaky sea salt. Let them cool on the baking sheets for about 2 minutes before transferring to wire racks.