Brown the Butter: Melt 1 cup unsalted butter over medium heat, stirring constantly until golden brown and nutty. Let cool slightly.
Mix Wet Ingredients: In a large bowl, whisk together 1 cup dark brown sugar, ½ cup granulated sugar, 2 eggs, 1-2 teaspoons corn syrup and 2 teaspoons vanilla until smooth. Slowly mix in cooled brown butter.
Combine Dry Ingredients: In a separate bowl, whisk together 2½ cups all-purpose flour, 1 teaspoon baking soda, and ½ teaspoon salt. Gradually fold into wet ingredients until just combined.
Add Mix-Ins: Fold in 1 cup toffee bits and 1 cup chocolate chips. If using an electric mixer up to this point, fold these in by hand so they don't get pulverized.
Chill: Let your cookies chill in the refrigerator for at least 3 hours up to 24 hours.
Scoop & Bake: Once chilled, scoop and roll dough into slightly larger than golf ball size portions and onto a lined baking sheet. Give your cookies room so 2 to 3 inches in between them. Bake at 350°F for 12–14 minutes, until edges are golden and centers are soft. While your cookies are baking, pop your cookie batter back in the fridge or freezer for the 12 minutes it takes for your cookies to bake in the oven.
Finish: Immediately sprinkle with more toffee bits, flaky sea salt if desired, or top with M&M style candies for color and crunch.
Cool: Let cookies cool 5 minutes on the sheet, then transfer to a wire rack.