"Puff pastry takes longer to cook that you think," says chef Alex Guarnaschelli. "It looks browned and ready on the outside but still be underdone in the middle. So let it cook! Dusting it with sugar for baking (or even running it under the broiler with sugar once cooked) caramelizes the outside of the pastry…So tasty! When you cut the millefeuille, use a serrated knife and a sawing motion to cut even (and fairly neat) portions. Or just put in the center of the table with some forks and let things happen as they may." Slideshow: More Raspberry Recipes
How to Make It
In a medium bowl, gently toss the raspberries with 2 tablespoons of the sugar and the zest and juice from the lemon. Refrigerate for 30 minutes.
Meanwhile, line a rimmed baking sheet with parchment paper. Unfold the pastry on a flat surface, roll it to a 12-by-15-inch rectangle and cut into three even 12-by-5-inch rectangles. Transfer to the prepared baking sheet and freeze for 20 to 25 minutes.
Preheat the oven to 425 degrees F. Dust the puff pastry with 2 tablespoons of the sugar. Cover the pastry with a wire baking rack to prevent puffing as the pastry cooks. Bake until golden brown, about 22 to 25 minutes. Remove the rack and let cool.
In the bowl of a stand mixer fitted with the whisk, beat the heavy cream, vanilla bean seeds and the remaining 1 tablespoon of sugar until soft peaks form, 3 to 5 minutes.
Place a spoonful of the whipped cream in the center of a rectangular platter and top with one sheet of puff pastry. Spoon over half of the whipped cream in the center leaving about 1 inch clear around the edges. Arrange about 1/3 of the fresh raspberries on top of the cream in rows. Top with another sheet of pastry and repeat with the remaining cream and another 1/3 of the raspberries. Add the last piece of pastry glaze-side up. Top with the remaining raspberries and serve immediately.