01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Roll out puff pastry on lightly floured surface to smooth seams. Cut into 12 equal rectangles (approximately 2.5 x 4 inches). Transfer to prepared baking 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 wire rack.
04 - In medium saucepan, whisk together milk, cream, sugar, and cornstarch until smooth.
05 - In separate bowl, whisk egg yolks. Gradually add small amount of hot milk mixture to yolks while whisking constantly to temper. Return yolk mixture to saucepan.
06 - Cook over medium heat, stirring constantly, until thickened and just starting to bubble (approximately 5 minutes). Remove from heat and stir in vanilla and butter until smooth.
07 - Transfer cream to bowl, cover surface directly with plastic wrap, and refrigerate until cold (at least 1 hour).
08 - Place one pastry rectangle on plate. Pipe or spoon vanilla cream over top, then scatter with fresh raspberries. Repeat with second layer of pastry, cream, and raspberries. Top with 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 before serving.