Brown the Butter: In a medium saucepan, melt the unsalted butter over medium heat, stirring constantly until golden brown and nutty, about 5-6 minutes. Transfer to a bowl, mix in espresso powder, and refrigerate until solidified, 1-2 hours.

Prepare the Dough: Beat the solid brown butter with granulated and brown sugars until light and fluffy, 3-4 minutes.

Add Egg and Vanilla: Mix in the egg and vanilla extract on low speed until well combined, about 30 seconds.

Mix Dry Ingredients: Sift in flour, baking powder, baking soda, and salt. Stir on low speed until just combined.

Fold in Toffee Bits: Gently fold in the toffee bits into the dough evenly.

Chill the Dough: Portion the dough into large balls and refrigerate for at least 2 hours, preferably overnight.

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

Bake the Cookies: Place dough balls on the prepared baking sheet and sprinkle with flaked sea salt. Bake for 11-13 minutes until edges are golden but centers are soft.

Cool and Serve: Cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.