• Preheat the oven to 350℉/176℃. Line the baking trays with parchment paper and set aside.

Brown the butter

• In a medium saucepan over medium-high heat, melt the butter. As it melts, it will begin to foam and sizzle. Stir constantly for 2–3 minutes to prevent burning. The butter is ready when it turns a deep amber color, brown specks form at the bottom, and it develops a rich, nutty aroma (about 3–5 minutes total). Transfer it to a heat-safe bowl or glass and let it cool for 15–20 minutes while you prepare the remaining ingredients.
  3/4 cups unsalted butter

Make the dough

• Mix together the flour, baking soda, cornstarch, cinnamon and salt. Set aside.
  1 3/4 cups + 1 tbsp all-purpose flour, 1 tsp baking soda, 1 tbsp cornstarch, 1/4 tsp cinnamon, 1/4 tsp salt
• In another bowl, add the slightly cooled brown butter, brown sugar, white sugar, and espresso. Whisk together until combined.
  3/4 cups unsalted butter, 1/2 cup white sugar, 1/2 cup dark brown sugar, 2 tsps espresso powder*
• Add in the egg, yolk, and vanilla. Whisk together, scraping the bowl as needed.
  1 large egg, 1 large egg yolk, 1 tbsp vanilla
• Add the dry and wet ingredients together. With a spatula, gently mix just until a soft cookie dough forms.
• Mix in the toffee bits and mini chocolate chips.
  1/2 cup toffee bits, 1/2 cup mini chocolate chips
• Scoop 6 cookies per tray, spacing them evenly apart from one another. Bake for 14-18 minutes or until the edges are set and crisp and the middles are slightly puffy and underbaked.
• Allow to cool, then enjoy!