Drain off and reserve the juice from the prunes, then press each prune gently between your fingers to slip out the stone. Tip the prunes with the apple juice into a food processor or blender (there should be no more than 100ml juice). Add the brandy, if using, then process the fruit to a purée.
In a small bowl, microwave the chocolate on High for 1-2 mins until melted. Stir in the orange zest and allow to cool slightly. Divide the yogurt between two bowls and stir the melted chocolate into one of them.
Layer alternate spoonfuls of the three mixtures in four dessert tumblers, starting with a layer of prune purée, then natural yogurt followed by the chocolate yogurt mix. Finish with prune purée and top this with natural yogurt. If you like a marbled effect, swirl the layers together using a skewer.
Chill in the fridge for at least 1 hr before serving, then scatter over the pistachio nuts to decorate.