Line a rimmed baking sheet with parchment or wax paper and grease it with butter. Set aside until toffee is ready to pour.
Melt butter, sugar, and salt in saucepan stirring constantly until butter is melted.
Bring mixture to a boil then stop stirring (except maybe a few times) and keep it boiling until candy thermometer reaches ~285-290º.
As soon as toffee mixture reaches the right temperature, pour in greased baking sheet.
Sprinkle chocolate chips all over hot toffee and let it sit for a few moments until it softens. Once it softens, spread melty chocolate chips around toffee until surface is covered.
If desired, add crushed nuts on some or all of the chocolate layer. (I like to do half with and half without.)
Place pan in fridge for about an hour until it sets and then it take out and either break into pieces or use a sturdy knife to score toffee and cut into squares.