• Brown the butter: Heat the butter in a light colored skillet over medium-high heat. It will come to a boil. Continue cooking, stirring occasionally, until the solid particles at the bottom turn golden brown. Keep an eye on it carefully towards the end, it can burn quickly. Immediately pour into a large mixing bowl to stop the browning. Let cool about 10 minutes so the sugars don’t melt when mixed into it.
• Whisk together the flour, cornstarch, baking soda, and salt in a bowl.
• Whisk sugars into cooled browned butter. Whisk in vanilla and eggs until thoroughly combined.
• Pour dry ingredients into bowl and stir until no streaks of flour remain.
• Stir in toffee bits.
• Cover with plastic wrap, pressing the plastic down on the surface of the dough. Chill for at least 3 hours and up to 3 days. If you chill longer than 3 hours, let dough sit at room temperature for about 30 minutes so the dough is easier to scoop.

To bake the cookies:

• Preheat the oven to 350° F. Line a baking sheet with parchment paper.
• Roll dough into balls, about 2 tablespoons each (40g). Place 2 inches apart on baking sheet.
• Bake on the middle rack until the edges are set and just beginning to turn golden brown, about 9 minutes. The tops of the cookies should still be quite soft when you remove them from the oven, and may even appear slightly underbaked in the center. They will firm up while cooling.
• Let rest on baking sheet for 5 minutes, then transfer to a cooling rack.
• Store in an airtight container for up to 5 days.