Heat the butter and brown sugar over medium in a heavy bottom pan.
Spray an 8x8 pan with pan spray or grease with butter.
Stir the butter and sugar while it melts, and then once it is fully combined allow it to start to boil. Stir once and then don't touch it other than to take the temp.
Boil until it reaches the hard crack stage at 310.
Pour immediately into greased pan. Allow to cool for 1 minute.
Sprinkle chocolate over and place a cookie sheet on top of the pan to help the chocolate melt. Use an offset spatula to spread the chocolate evenly.
Gently score the toffee into squares. You don't need to cut all the way through, you are just helping to give it a guide as to where to break apart. You can skip this if you prefer irregular pieces of toffee. I kind of like how they look when just randomly broken up.
Allow to fully cool. Once it is mostly cooled you can put it in the fridge to speed it up.
Break into pieces. Stores well at room temp or in the fridge or freezer.