Brown the Butter: In a medium saucepan over medium heat, melt 1/2 cup of unsalted butter. Stir continuously for about 5-7 minutes until it turns a deep golden brown and develops a delightful nutty aroma. Once browned, remove from heat and let it cool to room temperature.

Mix Dry Ingredients: While the brown butter cools, whisk together 1 1/2 cups of all-purpose flour, 1 teaspoon of instant espresso powder, 1/2 teaspoon of baking soda, and 1/2 teaspoon of fine sea salt.

Cream Sugars and Butter: In a stand mixer, cream together the remaining 1/2 cup of room temperature unsalted butter with 3/4 cup of brown sugar and 1/2 cup of granulated sugar until light and fluffy.

Incorporate Wet Ingredients: Slowly blend in the cooled brown butter and 1 teaspoon of pure vanilla extract, then add 1 large egg and 1 egg yolk until just combined.

Combine Mixtures: Gradually add the flour mixture to the wet ingredients, mixing on low speed until just combined. Gently fold in 1 cup of coarsely chopped bittersweet chocolate and 1/2 cup of toffee bits.

Chill Dough: Scoop the cookie dough onto lined baking sheets. Cover with plastic wrap and chill the dough in the refrigerator for at least 30 minutes or overnight.

Bake: Preheat your oven to 350°F (175°C). Sprinkle the chilled dough balls lightly with flaky sea salt and bake for 14-16 minutes until edges are golden brown.

Cool: Remove cookies from the oven and allow them to cool on the parchment paper for 10 minutes before transferring to a wire rack.