Make a white chocolate ganache with the chocolate, cream and rosewater, using the method (this makes twice that amount).
Turn the cake upside down so the flat base becomes the top and split it in half. Fill with a layer of the ganache, then with the raspberries, and top with the second layer of cake. Spread the top and sides with the remaining ganache.
Press the roasted pistachios all around the side of the cake and, when ready to serve, decorate the top with the edible flowers tightly packed together. Stunning.