Combine all the ingredients except the chocolate in a heavy-bottomed saucepan and bring to the boil.
Simmer, stirring occasionally, until the temperature reaches 125°C. It is easiest to do this using a cooking thermometer.
Otherwise, test the toffee by dropping a little of the mixture into a glass of cold water. If it makes a round, medium-soft ball, the toffee is ready. It will take about 30-40 minutes for the toffee to reach the right temperature.
Pour the toffee into a parchment-lined pan and leave to set. Cut into pieces.
Melt the chocolate over a water bath and dip the toffees in the chocolate. Allow to cool on a rack.
If you like, wrap the toffees in pretty paper.