In a heavy saucepan, bring all the ingredients to a boil, except for the butter, stirring to dissolve the sugar. Attach a candy thermometer in the centre of the pan and simmer, without stirring, until the thermometer reads 112 °C (234 °F). Remove from the heat and place the pan in an ice water bath. Let cool, without stirring, until the thermometer reads 43 to 50 °C (110 to 122 °F) or 20 to 30 minutes.
