In a dry large deep heavy skillet cook the sugar over moderately high heat, stirring constantly with a fork, until it is melted completely and a deep golden caramel. Remove the skillet form the heat, into the side of the skillet carefully pour 1/2 cup hot water, a little at a time, and cook the mixture over moderate heat, stirring, until the caramel is dissolved. Add the bourbon and simmer the sauce for 2 minutes. Remove the skillet form the heat, let the sauce cool slightly, and pour it into a heatproof pitcher. The sauce will thicken as it cools. To return the sauce to a liquid state set the pitcher in a saucepan of barely simmering water and stir the sauce until it reaches the desired consistency. Serve the sauce warm over ice cream.
