Heat the oven to 190C and set the rack in the centre. Lightly grease the base and sides of a 23-24cm fluted, loose bottomed tart or flan tin.
Roll the pastry out to 3mm thick and use to line the base and sides of the prepared tin. Trim the edges. Refrigerate for 30 minutes. Bake blind for 15 minutes (see instructions below). Remove the baking blind material and return to the oven for a further 3-4 minutes to ensure the base is well cooked. Set aside.
Place the prunes, butter, sugar and half the rum in a saucepan and place over a low heat, stirring until the prunes are plump, moist and mushy. Stir in the remaining rum and spread over the base of the cooked tart base.
Beat the butter, sugar and essence together until the mixture is pale and the texture is well whipped. Beat the eggs and add the egg mixture to the creamed mixture in small batches, beating well with each addition.
Stir in the cornflour, milk, lemon rind, lemon juice and ground almonds. Note that the mixture is likely to separate and look a bit curdled — this is normal. Spread over the prunes.
Bake at 160C oven for 35-40 minutes or until the top has set and is golden. Serve warm, dusted with icing sugar.
To bake blind:Line the pasty-lined tart tin with baking paper. Fill with baking blind material sufficient to cover the base and up around the sides of the tart. For baking blind material, you can use ceramic or metal beads purpose-made for the job, which are available at most cook shops and supermarkets, or use dried beans and/or rice. Do not throw them out, allow them to cook and store in an airtight container. They can be used over and over again.