Caramel and Scotch Sauce

Cook sugar in a dry 2-quart heavy saucepan over moderately low heat, stirring slowly with a fork until sugar is melted and pale golden. Cook caramel, without stirring, until deep golden, then remove pan from heat. Carefully add boiling water and Scotch (mixture will bubble up and vigorously steam), then cook over low heat, stirring with a wooden spoon, until caramel is melted and sauce is smooth. Serve warm.