Brown the butter in a medium saucepan over medium heat, stirring occasionally for 5-8 minutes until it turns a rich, nutty brown.

Pour the browned butter into a mixing bowl and let it cool for 10-15 minutes.

In the bowl with cooled brown butter, whisk together the light brown sugar and granulated sugar until smooth. Add eggs one at a time, mixing well. Stir in vanilla extract and instant espresso powder.

In a separate bowl, whisk together the all-purpose flour, baking soda, and salt.

Gradually fold dry mixture into wet ingredients until just combined.

Fold in toffee bits and chopped nuts if using.

Cover dough with plastic wrap and refrigerate for at least 30 minutes.

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

Drop rounds of dough onto prepared sheets, spacing them about 2 inches apart, and flatten slightly.

Sprinkle tops with flaky sea salt before baking.

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

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