Brown the butter by melting unsalted butter over medium heat, stirring frequently until it turns golden brown (5-7 minutes). Mix in espresso powder and chill for 1-2 hours until firm.

Beat browned butter with brown and granulated sugars for 3-4 minutes until light and fluffy. Add room temperature egg and vanilla extract, mixing until smooth.

Whisk together all-purpose flour, baking powder, baking soda, and salt in a separate bowl. Gently mix these dry ingredients into the wet mixture until just combined.

Fold in the Heath toffee bits gently, ensuring even distribution without vigorous mixing.

Scoop dough into balls on a baking sheet lined with parchment, then cover with plastic wrap and refrigerate for at least 2 hours or up to 24 hours.

Preheat oven to 350°F (175°C). Line additional sheets with parchment and space the chilled dough balls. Sprinkle lightly with flaked sea salt and bake for 11-13 minutes.

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