Brown the Butter: Melt 1 cup of unsalted butter in a medium saucepan over medium heat. Stir for about 5-8 minutes until browned.

Mix Sugars and Butter: Add ¾ cup light brown sugar and ¾ cup granulated sugar to the cooled butter. Blend until smooth and creamy.

Incorporate Eggs and Extract: Add 2 large eggs one at a time, whisking after each, then stir in 1 teaspoon of vanilla extract and 1 tablespoon of instant espresso powder.

Combine Dry Ingredients: In a separate bowl, whisk together 2 ¼ cups flour, 1 teaspoon baking soda, and ½ teaspoon salt. Gradually mix into the wet ingredients.

Add Toffee and Nuts: Fold in 1 cup toffee bits and ½ cup chopped walnuts or pecans.

Chill Dough: Cover with plastic wrap and refrigerate for at least 30 minutes or up to 24 hours.

Prepare to Bake: Preheat oven to 375°F (190°C) and line baking sheets with parchment paper.

Shape and Bake: Scoop dough onto baking sheets, flatten slightly, sprinkle with flaky sea salt, and bake for 9-11 minutes.

Cool: Let cookies rest on baking sheets for 5 minutes before transferring to wire racks to cool completely.