01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Roll out the puff pastry on a lightly floured surface to smooth the seams. Cut into 12 equal rectangles (about 2.5 x 4 inches). Transfer to prepared sheet and prick each piece several times with a fork.
03 - Place another sheet of parchment on top and weigh down with a second baking sheet to keep pastry flat. Bake for 15–18 minutes, until golden and crisp. Cool completely on a wire rack.
04 - In a medium saucepan, whisk together milk, cream, sugar, and cornstarch until smooth.
05 - In a separate bowl, whisk egg yolks. Gradually add a little of the hot milk mixture to the yolks to temper, then pour yolk mixture back into the saucepan.
06 - Cook over medium heat, stirring constantly, until thickened and just starting to bubble (about 5 minutes). Remove from heat and stir in vanilla and butter until smooth.
07 - Transfer cream to a bowl, cover surface with plastic wrap, and chill until cold (at least 1 hour).
08 - Place 1 pastry rectangle on a plate. Pipe or spoon vanilla cream over, then scatter with raspberries. Repeat with a second layer of pastry, cream, and raspberries. Top with a third pastry rectangle.
09 - Dust generously with powdered sugar and garnish with mint leaves if desired. Repeat to make 6 Napoleons. Serve immediately or refrigerate up to 2 hours.