Brown the Butter: In a medium saucepan, melt butter over medium heat until foamy. Cook until brown and nutty, about 5-7 minutes. Stir in espresso powder and cool for 1-2 hours.

Prepare the Dough: Combine cooled brown butter, brown sugar, and granulated sugar using a stand mixer. Beat until light and fluffy, about 3-4 minutes.

Add Egg and Vanilla: Mix in egg and vanilla extract. Beat until fully blended, about 1-2 minutes.

Incorporate Dry Ingredients: Gradually add flour, baking powder, baking soda, and salt. Mix until just combined.

Fold in Toffee Bits: Gently fold in toffee bits until evenly distributed.

Chill the Dough: Scoop the dough into balls and refrigerate for 2 hours or up to 24 hours.

Preheat Oven: Preheat your oven to 350°F (175°C). Prepare a baking sheet with parchment paper.

Bake Cookies: Place dough balls on the baking sheet, sprinkle with sea salt, and bake for 11-13 minutes.

Cool and Serve: Allow cookies to cool for 5 minutes on the baking sheet, then transfer to a wire rack.