Preheat the oven to 220°C, 425°F, Gas 7. Divide the puff pastry into four equal pieces, as each piece will be rolled and baked separately.
On a work surface lightly dusted with flour, roll out each piece of puff pastry to 20cm x 32cm (8in x 13in), making sure it is nice and thin – about 2mm. Trim the edges with a knife dipped in hot water (to give a nice neat edge) before carefully lifting the puff-pastry piece into the tin. Prick the pastry all over with a fork and chill it in the fridge for 10-15 minutes.
Bake in the oven for about 7-8 minutes, or until it is crisp and golden in colour. (The puff pastry will shrink a little while baking.)
Remove the pastry from the oven and allow it to cool on a wire rack while you roll out and bake the next puff-pastry layer in the same way. You may need to do this in batches, but, once the puff pastry has been cut, it will sit perfectly in the fridge while you are baking the other sheets. Repeat until all four puff-pastry layers are baked and have been set aside to cool.
In a bowl, whip the double or regular cream, whichever you're using, until it is thick, then place it in the fridge if you are not using it immediately.
To assemble the millefeuille, place one puff-pastry layer on a serving plate and spread it with a third of the whipped cream, then cover the cream with half of the raspberries (I like to save the best-looking raspberries for the very top).
Carefully spread the raspberry jam over the second puff-pastry layer, then place this layer on top of the raspberries on the previous layer. Spread with another third of the whipped cream. Add another layer of puff pastry on top of the whipped cream, then spread the remaining cream and raspberries on top of this.
Finish with the fourth and final layer of puff pastry, then dust liberally with icing sugar and add the extra raspberries for decoration.
Bring to the table to serve, cutting into squares as neatly as possible.