Begin by placing 1 cup of unsalted butter in a light-colored saucepan over medium heat. Stir occasionally, allowing the butter to melt and bubble. Watch closely as it turns a rich, nutty brown, about 5-8 minutes. Once browned, remove from the heat and pour it into a heatproof bowl to cool slightly.

In a large mixing bowl, whisk together the cooled brown butter with 1 1/4 cups of light brown sugar and 1/2 cup of granulated sugar until the mixture is creamy and smooth.

Next, crack in 2 large eggs one at a time into the sugar mixture, blending well after each addition.

Stir in 1 tablespoon of vanilla extract and 1 tablespoon of instant espresso powder, folding them into the batter until fully incorporated.

In a separate bowl, whisk together 2 1/2 cups of all-purpose flour, 1 teaspoon of baking soda, and 1 teaspoon of salt.

Gently fold in 1 cup of toffee bits and optionally, 1/2 cup of chopped walnuts or pecans.

Wrap the cookie dough in plastic wrap or transfer it to an airtight container and chill in the refrigerator for at least 30 minutes.

Preheat your oven to 375°F (190°C) and line two baking sheets with parchment paper.

Lightly flatten each cookie dough mound with your fingers to encourage even baking. Just before placing them in the oven, sprinkle a pinch of flaky sea salt on top of each cookie.

Bake the cookies in the preheated oven for 9-11 minutes until the edges turn a golden brown and the centers are still soft and slightly underbaked.

Allow the cookies to cool on the baking sheets for a few minutes before gently transferring them to a wire rack to cool completely.