Wash plums, halve, stone and cut into thin slices. Cut the vanilla pod lengthwise and scrape out the pulp.
Caramelise the sugar in a pan until golden. Stir in butter and lemon juice. Add plums and vanilla pulp. Simmer for about 5 minutes while stirring until the caramel is completely dissolved and reduce to a syrupy consistency.
Remove plums from the heat and let them cool off lukewarm.
Wash the lemon balm and shake dry. Pluck off the leaves and cut into fine strips. Arrange the plums with 1 scoop of ice cream on each. Sprinkle with lemon balm and dust with icing sugar.