Pre-heat the oven to 300 degrees. Whisk the yolks into the cooled milk mixture and strain through a fine sieve into a 9 x 13-inch baking dish. Place the dish in a larger roasting pan and add enough hot water to reach halfway up the side of the dish. Bake custard for 3/4 to 1 hour until a knife inserted in the center comes out clean. let cool; cover and chill for several hours or overnight. To assemble: Transfer pastry to a flat surface. Using a ruler measure out three 3 1/2 x 11-inch strips. With a serrated knife carefully cut the pastry using a gentle sawing motion. With a paring knife cut the custard into two 3 1/2 x 11-inch strips and cut each strip in half crosswise. With a long, wide offset spatula transfer a piece of custard onto a strip of pastry, covering half of it. Transfer a second piece of custard to fill the first layer. Top with a second strip of pastry, smooth side up, and repeat process. If necessary, smooth the sides of the napoleon with a knife so that they are stacked neatly.