Place butter and sugar in a heavy-duty sauce pan. Stir once in a while until it starts to reach high heat. Right after you stir the mixture, dip silicone pastry brush in water and run around the side of pan to dissolve the sugar crystals.
Continue to stir and watch closely as candy thermometer registers between 295-300°.
Remove from heat and pour immediately onto the 2 pans.
Add 2 broken up chocolate bars over each pan of the toffee and cover with foil. Let melt for a few minutes. Remove foil and spread melted chocolate with off-set spatula.
Sprinkle on nuts and put pans in the fridge to set for a few hours or overnight.