1. Heat sugar and 1/4 cup water in a medium heavy-bottomed saucepan over medium-high heat, stirring occasionally, until sugar has dissolved and syrup is clear. Continue to cook, without stirring, until syrup comes to a boil, occasionally washing down sides of pan with a wet pastry brush to prevent crystals from forming. Let syrup boil, gently swirling pan occasionally, until dark amber.

2. Remove from heat; whisk in lemon juice, liqueur (if desired), butter, and 2 tablespoons water. (Caramel will steam and spatter.) Serve warm.