Combine sugar and water in a small heavy saucepan. Bring to a boil over medium-high heat, stirring to dissolve the sugar. Cook, without stirring, until syrup turns light amber, 4 to 5 minutes. If the syrup isn't coloring evenly, gently swirl the pan. Remove from heat and carefully pour orange juice down the inside of the pan. (Stand back, as caramel may sputter.) When the foaming subsides, swirl in butter. Let cool slightly. Serve warm.