Preheat a moderate oven, Gas Mark 4, 180°C (350°F). Roll the pastry out to make four 10cm (4") diameter discs, then pleat the pastry all round the edge. Carefully arrange the sliced rhubarb on top of each pastry disc, making sure there are no gaps, then brush with the melted butter and sprinkle the sugar over. Cook in the preheated oven for 30 minutes. To make the sauce, whip the cream then whisk in the sugar and whiskey. When the rhubarb is tender and nicely caramelised on top, serve on heated plates with the whiskey cream and a sprig of fresh mint to garnish.