Line a 8" x 8" pan with parchment paper draped over each long side and set aside. Place the butter in a large saucepan over medium heat.
As the butter melts, stir constantly keeping an eye on the changing color of the butter. The butter will begin to foam up and bubble and you will notice it change color slightly from yellow to very light brown. The butter will burn if left for too long, so be on the lookout for a caramel/amber color.
Once the butter has browned, add the marshmallows and reduce the heat to low, stirring frequently. When the marshmallows are almost completely melted, add the puffed rice cereal and stir until coated. Remove from the heat.
Fold ½ a cup of the cacao nibs, all of the toffee bits and brown sugar into the rice cereal mixture until well incorporated. Pour the marshmallow-coated puffed rice cereal mixture into the prepared pan and, using a silicone spatula, spread into an even layer. Sprinkle the remaining ¼ cup of cacao nibs over the surface evenly.
Allow the rice krispie treats to cool to room temperature before cutting into even-sized squares. In a rush to stuff your face? Place the pan in the fridge for 10 minutes to allow the marshmallow to cool.
The rice krispie treats will keep for up to 3-4 days stored in an airtight container at room temperature.