Heat the oven to 425°F. On a lightly floured surface, roll the pastry to a 13x14-inch rectangle. Prick the pastry all over put the pastry on a baking sheet that is at least 13x14 inches (lay a sheet of parchment on top if you have any) and lay a cooling rack or another heavy baking sheet on top so that it’s perfectly flat on the pastry and there are no gaps between the layer of pastry and the rack or sheet on top. Bake until the pastry is very deep golden brown all over, 15 to 25 minutes; it shouldn’t puff at all except possibly around the edges abit. Remove the rack and parchment, slide the pastry off the sheet, and let cool on a rack.
Meanwhile, put the chocolate in a stainless-steel bowl set over a saucepan half-full of water. Bring the water to a bare simmer and melt the chocolate. Remove the bowl from the heat. In one pour, add 2/3 cup of the cream. With a whisk or a rubber spatula, gently stir until the mixture is smooth and glossy (it will look curdled at first). Stir in the vanilla and salt. Pour 3/4 cup of the chocolate mixture into a measuring cup and reserve it as the sauce.
Whip the remaining 1-1/3 cups cream until it forms soft peaks and then slowly pour in the remaining chocolate mixture (you should have about 1/4 cup) while continuing to beat. Stop when the cream just holds firm peaks; don’t overbeat or the cream will look and feel lumpy.
When the pastry is cool, using a sharp knife, trim the edges all around to make an even 12-inch square. Cut the square into sixteen 3-inch squares. Put eight pieces on eight dessert plates. Sift the confectioners’ sugar, if using, over the remaining pieces. Spoon a generous 1/3-cup of the chocolate cream onto each bottom piece and arrange the raspberries in three neat rows on top. Top with the remaining pastry pieces. Loosen the chocolate sauce to a flowing consistency by stirring in a few tablespoons of very hot water. Drizzle the chocolate sauce in a zigzag pattern over each napoleon and serve, passing more sauce at the table.
You’ll have a lot of chocolate sauce left over; refrigerate it for up to two weeks or freeze it for up to two months.