Beat the butter and sugar with an electric mixer in a large bowl until smooth. Add the egg yolks one at a time, allowing each egg to blend into the butter mixture before adding the next. Beat egg whites until foamy in a large glass or metal mixing bowl, continuing to beat until soft peaks form. Lift your beater or whisk straight up: the egg whites will form soft mounds rather than a sharp peak. Mix the egg whites into the egg yolk mixture. Stir in the rum, salt, mace, allspice, cloves, nutmeg, and cardamom.
To use the mix, combine 1 ounce rum and 1 cup hot milk in a mug. Stir in 1 tablespoon of the Tom and Jerry mixture.