Preheat the oven to 450F. Lightly grease a large baking sheet and sprinkle with water. Roll out the dough to a rectangle about 1/8-inch thick, just slightly larger than the baking sheet. Loosely roll up the dough on the rolling pin and then unroll it over the baking sheet. Trim the edges with a sharp knife. Prick the surface of the dough all over with a fork to prevent it from rising too much. Bake until golden brown and crisp, about 25 minutes. Cool on a wire rack. To make the pastry cream put the milk and vanilla in a heavy-bottomed saucepan and bring to a boil. Combine the egg yolks and sugar in a mixing bowl and beat until foamy. Whisk in the flour and cornstarch. Whisk in the hot milk and return the mixture to the saucepan. Bring to a boil over a medium heat, whisking constantly. Reduce the heat and simmer 5 minutes, whisking constantly to reach all parts of the pan. Spread the cream in a shallow dish and dot with butter to prevent a crust from forming. Allow to cool. To assemble the NapolÄon, trim the edges of the pastry with a serrated knife. Cut lengthwise into 3 equal strips. Spread one strip with half of the pastry cream. Cover with a second strip of pastry and spread with the remaining pastry cream. Place the remaining strip on top of the final layer of pastry cream. Smooth off any pastry cream on the sides with a palette knife. Sift confectioner╒s sugar liberally over the NapolÄon.
