Step 1: Brown the Butter

1. Place unsalted butter in a saucepan over low heat. Cook while stirring or swirling the pan frequently.
2. The butter will foam and bubble; move the foam aside occasionally to monitor the color. When the butter turns amber/brown and has a nutty aroma, it’s ready (about 7–10 minutes).
3. Transfer the browned butter (including the browned bits) to a heatproof bowl and let it cool and solidify at room temperature (about 1 hour).

Step 2: Prepare Dry Ingredients

1. In a medium bowl, whisk together the flour, baking soda, cornstarch, espresso powder, and kosher salt. Set aside.

Step 3: Mix Wet Ingredients

1. In a stand mixer fitted with the paddle attachment, cream the cooled browned butter with dark brown sugar and granulated sugar on medium-high speed until light and fluffy.

Step 4: Add Eggs and Vanilla

1. Add eggs one at a time, mixing well after each addition.
2. Stir in the vanilla extract.

Step 5: Combine Dry and Wet Ingredients

1. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Avoid overmixing.

Step 6: Fold in Toffee Bits

1. Gently fold in the toffee bits with a spatula.

Step 7: Chill the Dough

1. Cover the dough and refrigerate for at least 30 minutes to 1 hour to enhance flavor and texture.

Step 8: Bake the Cookies

1. Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper.
2. Scoop dough into 2-tablespoon-sized balls and place them 2 inches apart on the baking sheets.
3. Bake for 10–12 minutes or until the edges are golden and the centers look slightly underbaked.

Step 9: Cool and Finish

1. Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.
2. If desired, sprinkle flaky sea salt on top of the warm cookies for added flavor.