Heat slowly, stirring until butter melts, sugar dissolves, then increase the heat to bring mix to a boil.
If using sugar thermometer, boil to 160 degrees or test by dropping a small amount into a glass of cold water. If it is ready, it will be a small blob of toffee. It should also be slightly stretchy but breaks with pressure.
Pour into a butter sugar tin and leave to set. To break even pieces, mark toffee when it is partially set. Break into pieces when cool.