Melt the butter over medium to high heat for about 10 minutes, or until the butter smells nutty and is browned. The butter will foam before turning brown. Keep a close eye on the butter because it will go from brown to burned very quickly. When done, set aside.
In a bowl, add flour, baking soda, and salt. Mix.
In another bowl, add the brown and granulated sugars to the browned butter. Mix until well incorporated. Add the eggs and vanilla, mix.
Add the flour mixture to the wet ingredients. Mix until all ingredients are well combined. Add the toffee bits, chocolate chips and hazelnuts.
Chill the dough for about an hour. Before baking, let the dough return to room temperature. Preheat the oven to 350 degrees F. Bake for 8 to 10 minutes or until golden brown.