Stir egg yolks and lime juice vigorously in a 1 1/2-quart saucepan. Add 1/4 cup of the butter. Heat over very low heat, stirring constantly, until butter is melted. Add remaining butter. Continue heating, stirring vigorously, until butter is melted and sauce is thickinged. (Be sure butter melts slowly so that sauce will thicken without curdling.) Stir in lime peel. Serve hot or at room temperature. Cover and refrigerate any remaining sauce. Makes about 3/4 cup of Lime Hollandaise sauce.