Heat the milk almost to boiling point, stir in the softened gelatin until it is dissolved. Add the egg yolk-sugar mixture and stir over very low heat until the mixture coats the spoon. Remove from heat.
Beat the egg whites with a pinch of salt and remaining tablespoon of sugar until they stand in peaks. Add the rum and preserved ginger to the cooled custard, mixing well.
Fold in the egg whites, pour into a souffle dish or mold and refrigerate until set.