Joanne Chang's Chocolate Mousse Napoleon
1 quart chocolate ganache, cooled and slightly chilled but still fluid
1 8.5 oz. sheet (about 10-inches wide) store-bought frozen puff pastry, thawed in the refrigerator overnight or at room temperature for an hour
2 pints fresh raspberries, halved
4 mint sprigs
Preheat the oven to 425° conventional (400° convection) and have an oven rack in the center of the oven.
Reserve ½ cup ganache. In the bowl of a stand-mixer or using a hand-mixer, whip the remaining ganache on high-speed until it is thick and holds a peak, about 3 minutes. Do not over-mix. Place the whipped ganache in a large pastry bag fitted with a plain tip and reserve. (If no pastry bag is available, fill a gallon-size plastic storage bag and cut a small piece of the tip off with scissors).
Lay the puff pastry sheet out on a cutting board. Dock the dough, piercing it all over with a fork. With a sharp knife, cut the dough in half lengthwise then in thirds crosswise to form 6 rectangles. Transfer the dough to a sheet tray lined with parchment paper, spacing them about an inch apart. Top the dough with a second sheet of parchment paper. Place an inverted wire rack directly on top of the parchment paper. Place in the oven on the middle rack until the dough is slightly puffed, golden brown in color, and is cooked through, about 15 minutes. Transfer the puff pastry pieces to a wire rack to cool. Once fully cooled, using a serrated knife, split the rectangles into 12 halves.
To build the Napoleons: place a puff layer on the serving plate. Pipe a thin layer of whipped ganache (about 3 tablespoons), and top with a single layer of raspberries. Top with another puff pastry rectangle. Repeat with a second layer of mousse and raspberries. Top with a third puff pastry rectangle, exterior side up. Using a small, off-set spatula, glaze with about 2 tablespoons of the reserved ganache. Repeat to build 3 more Napoleons. Garnish with a mint sprig and serve immediately.