Beat egg yolks slightly in top of a double boiler. Add sugar, lemon juice, butter and salt; mix well. Cook over boiling water, stirring constantly, until thickened. Remove from water; cool to room temperature.
Mix grapes, pineapple, oranges, marshmallows and coconut in a large bowl. Fold whipped cream into cooled egg yolk mixture. Fold into fruit mixture. Cover and refrigerate at least 24 hours; the long chilling is necessary to mellow and develop flavors.