Brown the butter: In a medium sized saucepan, add the 1/2 cup of butter and melt it on medium heat. Once it melts, it will crackle, then become foamy – continue mixing the butter while this happens to prevent burning. The butter will go from melted to golden brown quickly, and you'll know it's almost done when the aroma of browned butter arrives. The whole process should take about 5 minutes! Once golden, remove from heat and transfer to a glass bowl to cool for 20 minutes while you prep the rest of the ingredients.
In a large bowl, whisk together the cooled browned butter, dark brown sugar, and white sugar.
1/2 cup butter, 1/2 cup dark brown sugar, 1/3 cup white sugar
Add in egg and vanilla and continue whisking.
1 egg, 1 tbsp vanilla
Toss in the flour, cornstarch, baking soda, baking powder, and salt. Using a spatula, gently fold until a thick dough forms.
1 1/3 cups all-purpose flour, 1/2 tsp baking soda, 1/2 tsp baking powder, 1/2 tsp cornstarch, 1/4 tsp salt
Toss in the toffee bits and walnut pieces. Mix in a few more times.
1/2 cup toffee bits, 1/2 cup walnuts
Preheat the oven to 350℉/180℃ and line a baking tray with parchment paper.
Scoop 2 tbsp worth of dough using a large ice cream scoop and place onto a baking sheet. Bake 6 cookies at a time.
Bake the cookies for 10-12 minutes or until the edges are a light golden brown colour. Mine took 11 minutes. The middle will still look soft, but will continue to bake from the warmth of the pan after it's out of the oven!
Cool the cookies on the baking tray for 10 minutes and then transfer to a cooling rack. Add some flakey sea salt on top (optional, but recommended), and enjoy!