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 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, then 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 chill until cold (at least 1 hour).
08 - Place 1 pastry rectangle on plate. Pipe or spoon vanilla cream over top, then scatter with 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.