Step 1

Preheat the oven to 375°F. Prepare two baking sheets by lining each with parchment paper. Set aside.

Step 2

In a stainless steel skillet (or another light-bottomed pan) over medium-high heat, melt the butter, swirling the pan occasionally. The butter will begin to foam, which signals the water in the butter is boiling off - watch closely at this stage. Tiny brown specs of milk solids will begin to appear at the bottom of the pan - constantly stir and scrape these off using a spatula so they don't stick. As soon as the butter turns chestnut brown and smells nutty, remove the pan from the heat.

Step 3

To the browned butter, add dark brown sugar, granulated sugar, salt, and vanilla. Whisk until the ingredients are fully incorporated.

Step 4

Next, whisk in the egg and egg yolk for 30 seconds, or until there are no lumps. Then, set a timer for 3 minutes before whisking another 30 seconds. Start the 3-minute timer again, whisking for 30 seconds after that. Repeat the 30-seconds on (mixing), 3 minutes off, a total of FOUR times. The dough will be a beautiful dark golden toffee color. (On one of the three minutes 'off', you could mix together the flour and baking soda.)

Step 5

In a medium bowl, add flour and baking soda. Whisk to combine and add this to the browned butter mixture, mixing until just combined.

Step 6

Finally, fold in the chocolate and toffee bits, reserving a few for the tops of the cookies, if desired.

Step 7

Using a 3 tablespoon scoop, portion cookies and place on the parchment-lined baking sheets. Leave about 2 inches between each cookie. Top with a few reserved chocolate and toffee chips, if adding.

Step 8

Bake each sheet of cookies separately for 10-14 minutes, depending on the size of your cookies. Remove from the oven and let cool completely before serving.