Brown the Butter: In a saucepan, melt the unsalted butter over low heat, stirring frequently. Cook until brown flakes form and it smells nutty, then let cool to room temperature for about one hour.

Whisk the Dry Ingredients: In a medium bowl, combine the flour, baking soda, cornstarch, espresso powder, and kosher salt. Whisk until well-mixed and set aside for later.

Cream the Butter and Sugars: In your stand mixer or using a hand mixer, beat the cooled, browned butter with both sugars on medium-high speed until light and fluffy, about 4-5 minutes.

Incorporate Eggs and Vanilla: Add one egg and the vanilla extract. Mix for about 30-60 seconds until fully combined, add the second egg, and mix again for another 30-60 seconds.

Mix in the Flour: With the mixer set to low, gradually add the flour mixture in three increments, scraping the bowl before adding the last portion. Stop when just a few streaks of flour remain.

Fold in Toffee Bits: Gently fold the toffee bits into your dough using a spatula, ensuring even distribution throughout.

Chill the Dough: Portion the dough into 24 equal scoops. Place them on a tray and chill in the refrigerator for at least one hour.

Preheat the Oven: Set your oven to 350°F (175°C). Line your baking sheets with parchment paper, making sure to leave about 2 inches of space between the cookie dough portions.

Bake the Cookies: Bake for 8-10 minutes until the edges turn lightly golden. Optionally sprinkle flaky sea salt on top.

Cool the Cookies: Let the cookies cool on the baking sheet for about 2 minutes before transferring them to a wire rack to cool completely.