In a heavy, nonreactive saucepan, beat the yolks with the sugar until well blended. Stir in the lime juice and butter. Cook over medium-low heat, stirring continually, until it starts to thicken into a sauce. If it starts to steam or boil, remove from heat and continue to stir. The sauce will resemble a thin hollandaise when cooked, and will coat the back of the spoon. If the sauce boils, it will curdle.