Cut the pastry into 6 even rectangles. Knock up and flute the edges with a sharp knife then transfer to a lightly greased baking sheet. Prick the centres of the pastries.
Cream the butter and sugar together, then mix in the ground almonds and egg yolk. Divide the mixture between the pastry rectangles, then spread into a thin layer, leaving a border of pastry around the edges.
Arrange the plums on top of the almond mixture, then bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 12–15 minutes until the pastry is well risen and golden. Loosen the bases of the tarts with a palette knife then allow to cool.
To make the custard, whisk the egg yolks, sugar, cornflour and saffron together in a bowl. Heat the milk in a saucepan until just boiling then gradually whisk into the egg yolks. Pour the milk mixture back into the pan then heat gently, stirring continuously, until almost boiling, and thickened and smooth. Tip back into the bowl and sprinkle with a little extra sugar to prevent a skin forming. Cover, allow to cool, then chill until required.
Serve the tarts and custard cold, or reheat if preferred and dust with a little sifted icing sugar.