Preheat the oven to 180℃/400℉ and line two large baking trays with baking parchment.
Roll out the puff pastry on a floured surface to a thickness of about 5mm (1/4in) – you’ll be splitting it into layers once baked.
Slice the pastry into twelve 7.5 x 10cm (3 x 4in) rectangles, trying to keep them fairly even, then place them spaced apart on the lined baking trays (you may need to bake the rectangles in batches). If using beat the egg and brush over the pastry. Bake in the oven for 10–12 minutes, until just golden, then remove and leave to cool.
Once the pastry is cool, whisk the cream until soft peaks form, by hand or with an electric handheld whisk. Take care if using an electric whisk, as it’s easy to overwhip the cream. Set aside.
Pull open the cooled puff pastry rectangles along the long side of the rectangle to split them in half: the top and bottom part should be separated. Spread a teaspoon of jam along the inside of one half and a good spoon of whipped cream on the other than before sandwiching together. Dust over some sifted powdered (icing) sugar and serve immediately!