To brown the butter: Place unsalted butter in a saucepan over low heat and cook until small brown flakes form, stirring frequently. The butter will turn an amber/brown color and will smell nutty and toasted when ready (about 7-10 minutes). Pour melted butter into a heat-proof bowl to cool and solidify back to room temperature (about 1 hour).

In a medium-sized bowl, whisk flour, baking soda, cornstarch, espresso powder, and kosher salt. Set aside.

Cream brown butter and both sugars on medium-high speed with a stand mixer fitted with the paddle attachment or a hand mixer until light and fluffy (4-5 minutes).

Scrape down the bowl and add one egg and vanilla. Mix until well combined (30-60 seconds). Scrape the bowl again, add the remaining egg, and mix another 30-60 seconds or until well combined.

With the mixer on low, add the flour mixture in three additions, scraping the bowl before the last amount of flour is added. Blend until just a few streaks of flour remain.

Fold in the toffee bits with a spatula.

Use a size 30 scoop (just over 2 tablespoons) to portion the dough into 24 equal scoops. Chill the dough for one hour.

Preheat oven to 350°F and line baking sheets with parchment paper or a silicone baking mat. Place cookie dough on prepared baking sheet, spacing about 2 inches apart.

Bake for 8-10 minutes, until edges are lightly browned and crisped. Drop the pan hard on the counter once to give the edges a little crinkle and deflate the cookie. If puffy, use the flat bottom of a cup to lightly press the cookies down. Sprinkle with flaky sea salt while still hot, if desired.

Allow to cool on the pan for 2 minutes and then transfer to a cooling rack to cool completely.