Heat oven to 200C/180C fan/gas 6. Unroll 1 pastry sheet, but leave it on the parchment. Place it on a baking tray, dust with 2 tbsp icing sugar, then cover with another sheet of parchment and another baking tray. Pour baking beans into the tray to weigh it down, or use empty cans. Bake for 30 mins, then check if the pastry is golden all over. If not, return for another 5 mins or so. Repeat with the other pastry sheet (or cook both together if you have enough trays). Once out of the oven, trim the edges with a large, sharp knife. Leave to cool.
Softly whip the mascarpone, cream, vanilla and remaining icing sugar, then transfer half to a disposable piping bag fitted with a 1.5cm round nozzle (or just snip off the corner). Pipe blobs of the mascarpone cream over the surface of one piece of pastry, right up to the edges. Top with half the fruit and dust with a little icing sugar.
Top with the second piece of pastry, more mascarpone cream (refilling the bag when you need to), the remaining fruit, a dusting of icing sugar and, finally, some mint leaves. Use a serrated knife to cut through the layers without squashing the whole tart. Best served within an hour or two, but leftovers will keep for up to two days in the fridge.