In 2-quart suacepan combine sugar, brown sugar and butter. Cook over low hear, stirring occasionally, until butter is melted (6-8 min). In large mixing bowl, combine cooked mixture with ice-cream, beat at medium speed, scraping bowl often until smooth (1-2 min). Store refrigerated up to 2 weeks or frozen up to one month. For each serving, fill mug with 1/4 cup mixture, 1 oz. rum and 3/4 cup boiling water, sprinkle with nutmeg.