In a saucepan, dissolve the cornflour in the milk. Add condensed milk, coconut milk and desiccated coconut. Place over a medium heat and simmer until thickened, stirring constantly with a wooden spoon.
Place a 24cm tube cake tin in a cold water bath. Pour in the milk mixture, then transfer to the fridge and chill for 3 hours, or until firm.
In a saucepan over low heat dissolve sugar in water, stirring constantly. Add prunes and cook without stirring, for 10 minutes until the syrup thickens. Remove from heat and let cool.
Unmould the blancmange on a platter before serving. Drizzle with syrup and garnish with the prunes. Serve cold or at room temperature.