Brown the butter by melting unsalted butter over medium heat, swirling occasionally for 5-8 minutes until it turns a rich, nutty brown color.

Combine cooled browned butter with light brown sugar and granulated sugar until smooth and creamy.

Incorporate two large eggs one at a time, mixing well after each addition, then add vanilla extract and espresso powder.

Prepare dry ingredients by whisking together all-purpose flour, baking soda, and salt in a separate bowl.

Gradually combine the dry ingredients into the wet mixture until just combined; avoid overmixing.

Fold in toffee bits and nuts until evenly distributed without overworking the dough.

Chill the dough for at least 30 minutes or up to 24 hours for enhanced flavor and texture.

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

Shape tablespoon-sized amounts of dough onto prepared sheets, flatten slightly, and sprinkle with flaky sea salt.

Bake for 9-11 minutes until edges are lightly golden and centers remain soft.

Cool on baking sheets for 5 minutes before transferring to a wire rack to cool completely.