Brown the butter by placing it in a medium saucepan over medium low heat.  Cook and stir or swirl butter until brown flakes form from the pan’s bottom and begin to smell nutty. Butter will foam and sputter.  Watch carefully to avoid burning.  When overall amber brown (about 5-9 minutes), remove butter from heat and pour into a bowl to cool and solidify.  Depending on your kitchen temperature, this can take up to 2 hours. If needed, butter can be placed in refrigerator to solidify.

Whisk flour, soda, cornstarch, espresso powder, and salt together in a. bowl.  Set aside.

In the bowl of a standing mixer, or using a hand-held mixer, cream solidified browned butter with brown sugar and granulated sugar until fluffy.  Scrape down the sides and bottom of bowl.  Beat in eggs and vanilla until well combined and fully incorporated.   Stir in dry ingredients until just combined. Stir in toffee bits by hand using a spatula.

Portion dough into balls using a #24 or #30 ice cream scoop (about 2-1/2 to 3 T. amounts) and place onto a parchment lined baking sheet.  Cover and chill in refrigerator for about 1 hour.

Preheat oven to 350 degrees F.  Portion dough about 2 inches apart on additional parchment lined baking sheets.  Sprinkle each dough ball with flaky salt.  Bake for 10-12 minutes, or until edges are lightly browned and crisp.  The centers may be slightly undone.  Remove baking sheets to wire rack to cool.  Store cookies in an airtight container for up to 5 days, or in a freezer safe container for up to 3 months.  Dough can also be frozen for up to 3 months as well.