Brown the unsalted butter in a medium saucepan over medium heat, stirring frequently for 5–7 minutes until deep golden brown.

In a large bowl, combine the cooled brown butter with granulated and brown sugars, mixing until smooth and creamy, about 2–3 minutes.

Add the eggs one at a time, mixing well after each until fully combined, about 2 minutes.

In a separate bowl, whisk together flour, baking soda, salt, and instant coffee until evenly combined.

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

Fold in the toffee bits gently with a spatula, ensuring they are distributed throughout the dough.

Cover the dough and refrigerate for at least 30 minutes or up to 3 days.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper, then scoop dough onto sheets spaced 2 inches apart.

Bake for 10–12 minutes until edges are golden brown and centers are slightly soft.

Sprinkle sea salt flakes on top, cool for 5 minutes on the sheet, then transfer to a wire rack to cool completely.