For the tart, preheat the oven to 200C/ 400F/gas 6. Line a square or rectangular baking tray with parchment paper.
On a lightly floured work surface, roll the pastry to a 0.5cm/⅛in thickness. Use the rolling pin to lift it onto the baking tray and trim to fit.
Roll out the marzipan so that it will sit on top of the pastry leaving a 3cm/1in clear border of pastry all the way round, then place it on top of the pastry.
Dot the plums on top of the marzipan, then sprinkle with the sugar (how much you use will depend on the sweetness of the plums; remember the marzipan is very sweet).
Brush the pastry border with the beaten egg then bake in the oven for about 20 minutes, or until the pastry is puffed up and golden-brown and the marzipan is nice and gooey.
Meanwhile, for the orange mascarpone, beat all the orange mascarpone ingredients together.
Serve slices of the hot tart with the cold mascarpone cream.