In a small saucepan over medium heat, cook butter, stirring frequently. After a few minutes it will bubble, then foam, and then quickly brown - at that point, turn off the heat and give it a quick stir. Pour into a large bowl and let cool.

In a medium bowl, whisk together flour, baking soda and kosher salt. Set aside.

In the large bowl with the brown butter, add brown sugar and granulated sugar. Use a hand mixer on medium speed to beat to combine (about 1 minute).

Add egg and vanilla extract and beat again until the mixture is thicker and combined (another minute).

With your mixer running on low speed, slowly add the flour mixture and beat until just combined.

Use a spatula to fold in the chocolate and toffee pieces.

Set the dough aside to rest and thicken (on the counter, not in the fridge) - at least 20 minutes, longer if you have the time.

Use a medium cookie scoop (about 1 1/2 tablespoons) to drop batter onto prepared baking sheets. They'll spread quite a bit as they bake, so leave plenty of room - I usually only do 5 cookies on each sheet, just to be safe.

Top each cookie with some extra chocolate pieces or toffee bits if desired, as well as a small sprinkle of flake sea salt.

Bake on the middle rack for 8 minutes or until the edges are golden, but the centers still soft.

Let cool on the baking sheet for 5-10 minutes (they will crinkle and set during this time), then transfer to a wire rack to cool completely.

Serve and enjoy!