According to mixologist Tonu Abou-Ganim in his book The Modern Mixologist, the key to success in making a superior Tom & Jerry cocktail is to use fresh batter. You want to be sure each drink is made and served while the batter is still light and fluffy.
For the Batter: Separate egg whites and yolks. In a large mixer, beat yolks until creamy, then transfer them to another bowl. Clean the mixer and add the egg whites and cream of tartar, then beat until stiff. Add powdered sugar and fold in creamed yolks. Mix until batter is light and fluffy.
For each drink: In a pre-heated mug, add 2 heaping ladles of batter (roughly 6 oz or 180 ml). Add rum and cognac. Top with hot water and dust with freshly grated nutmeg. Serve with a paddle or spoon.