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 (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 paper on top and weigh down with a second baking sheet to keep pastry flat. Bake for 15 to 18 minutes until golden and crisp. Cool completely on a wire rack.
04 - In a medium saucepan, whisk together milk, heavy cream, granulated sugar, and cornstarch until smooth.
05 - In a separate bowl, whisk egg yolks. Gradually add a small amount of the hot milk mixture to the yolks while whisking constantly to temper, then pour the yolk mixture back into the 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 extract and butter until smooth.
07 - Transfer cream to a bowl, cover surface directly with plastic wrap to prevent skin formation, and refrigerate until cold (at least 1 hour).
08 - Place 1 pastry rectangle on a serving plate. Pipe or spread vanilla cream over the surface, then distribute fresh raspberries evenly. 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 fresh mint leaves if desired. Repeat assembly process to create 6 Napoleons total. Serve immediately or refrigerate for up to 2 hours before serving.