In a medium saucepan, melt 2 sticks of unsalted butter over medium heat, stirring continuously until it darkens to a nutty brown, about 5-7 minutes. Let cool for 15 minutes.

In a large mixing bowl, combine cooled browned butter with ½ cup of granulated sugar and 1 cup of dark brown sugar. Whisk until smooth.

In a separate bowl, whisk together 1½ cups of all-purpose flour, 1 cup of bread flour, 1 teaspoon of baking soda, ½ teaspoon of baking powder, 1 teaspoon of fine sea salt, and 1 teaspoon of instant espresso powder.

Crack 2 large eggs and an egg yolk into the butter and sugar mixture. Add 2 teaspoons of vanilla extract and whisk until smooth.

Gradually fold the dry ingredients into the wet ingredients using a spatula until just combined. Gently fold in 10 ounces of chopped semisweet chocolate and 1 cup of toffee bits.

Wrap the cookie dough tightly in plastic wrap and refrigerate for at least 24 hours (1 hour minimum if short on time).

After chilling, let the dough sit at room temperature for approximately 1 hour. Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.

Using a cookie scoop or measuring 3-tablespoon portions, place the dough on baking sheets. Optionally freeze for 15 minutes to firm up.

Bake cookies for 12-14 minutes or until edges turn golden brown. Sprinkle with flaky sea salt and cool before transferring to a wire rack.