Preheat the oven to 200c/400f and line a baking sheet with some baking parchment.
First make your frangipane. Beat the butter and sugar together for a few minutes until light and creamy (you can do this in a mixer or by hand). Add the egg and beat again for a minute. Add the flour, baking powder, ground almonds and almond extract and mix for 2 more minutes. Put to one side.
Place your rolled out pastry on a lightly floured surface (or just leave it on the wrapping it comes in). Cut out four small circles approx. 6.5cm in diameter and four larger circles approx. 11cm in diameter.
Place the smaller circles on the baking tray and spread a tsp of frangipane on each one, leaving a small border.
Gently mix the raspberries with the jam and cornflour. Then pile 3 raspberries on top of each pastry base. Make sure you don't go over the layer of frangipane you've just pasted on.
Top each one 1.5tsp-2tsp more frangipane, and spread it over and around the raspberries - so the raspberries aren't showing. It's a bit of a balancing act, but don't worry too much if you get a bit of the jam sticking out.
Gently place one of the larger circles of pastry on top of one of the mounds. Seal the edges with your fingers and crimp if you want to (see picture). Repeat with the other three.
Give each pithivier a light egg wash, and make a hole in the top of each pithivier using a cocktail stick. Score some lines onto the pastry to decorate if you wish. Decorate the edges of your pastry with the almond slivers. Then place in the oven.
Cook at 200c/400f for 10 minutes, then turn down to 180c/350f and cook for another 10 minutes until golden brown and puffed up.
Take out of the oven and leave to cool slightly before eating.
These pastries will keep for about a day in a sealed container at room temperature..Nutritional Information is per serving.