Combine all of the ingredients in a heavy-bottom saucepan over medium heat. Heat slowly, stirring until the butter has melted and the sugar has dissolved.
Increase the heat and bring the mixture to a boil. Cover and gently boil for around 2 minutes.
Remove the cover and continue to boil, stirring occasionally, until the mixture reaches 300°F on a candy thermometer; it should take 10-15 minutes.
Pour the mixture into a buttered 6x6-inch pan, let the toffee cool to the touch and score it into 1-inch squares. Let the toffee cool completely; turn it out onto a board, bread the toffee into pieces and warp them individually in squares of waxed paper. Store at room temperature in an airtight container.