In a small saucepan, heat milk until small bubbles form. Drop in the vanilla pod, remove saucepan from heat, and set aside to cool until just warm. In a medium saucepan, stir together the flour, sugar, salt, and egg yolks. Beat for a moment, then gradually whisk in warm milk. Simmer over medium-low heat until the custard thickens, stirring constantly with a wooden spoon to prevent sticking. Transfer custard to a bowl, and allow to cool, stirring from time to time. Preheat oven to 400 degrees. Roll out the puff pastry into a single sheet about 1/2 inch thick; it should be the same size as a 17x14 inch baking sheet. Place the pastry onto the baking sheet, and prick all over with a fork. Bake in preheated oven for 28 minutes. Remove from oven, and cool pastry on the baking sheet. When the pastry has cooled completely, transfer it from the baking sheet to a hard surface, and cut lengthwise into three 4 to 5 inches wide strips. Spread one strip thickly with custard. Place a second strip directly over the first; spread the top evenly with jam. Cover with the third strip, and dust with confectioners' sugar. With a very sharp knife, cut into 8 rectangular portions.