In a medium saucepan over medium heat, melt the unsalted butter, swirling the pan occasionally until browned.

Transfer brown butter to a mixing bowl and cool for 10–15 minutes.

Whisk in the light brown sugar and granulated sugar until smooth, then beat in the eggs, vanilla extract, and espresso powder.

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

Gradually add dry ingredients to the wet mixture, stirring gently until just combined.

Fold in the toffee bits and optional chopped nuts.

Cover the bowl and refrigerate the dough for at least 30 minutes.

Preheat your oven to 375°F and line baking sheets with parchment paper.

Drop rounded portions of dough onto prepared sheets, flatten slightly, and sprinkle with flaky sea salt.

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

Let cookies rest on baking sheets for 5 minutes before transferring to a wire rack.