This recipe is from Tyler’s America’s Test Kitchen book. It is a great summer dessert - easy to make, tastes delicious, and looks pretty enough to serve to guests.
Adjust an oven rack to the middle position and heat the oven to 425 degrees. Cut the puff pastry into 12 rectangles, transfer to a rimmed baking sheet, cover with plastic wrap, and refrigerate for 15 minutes. Brush the rectangles with the beaten egg and sprinkle lightly with the granulated sugar. Bake until puffed and golden, about 15 minutes. Let the pastry rectangles cool for 15 minutes, then split them in half using the serrated knife.
Whip the cream with an electric mixer set on low until frothy, about 1 minute. Increase the speed to high and continue to whip until smooth, thick, and doubled in volume, 1 to 3 minutes. Add the pudding and vanilla to the whipped cream and whip until stiff, about 1 minute. Spread 1/8 cup of the pudding mixture over the bottom of each pastry square, then top with 1/4 cup berries. Spoon another 1/8 cup of the pudding mixture over the berries.
Gently press the pastry tops into the pudding mixture to adhere, then lightly dust with the confectioners’ sugar. Dollop the remaining pudding mixture attractively on top of each pastry before serving.
To Make Ahead: The puff pastry can be baked, cooled, wrapped in plastic wrap, and stored at room temperature up to 24 hours in advance. The cream and pudding mixture can be whipped together up to 2 hours before assembling the napoleons. After being assembled, the pastries will hold in the refrigerator for about 30 minutes before they begin to turn soggy.