01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Roll out thawed 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 fork to prevent puffing.
03 - Place another sheet of parchment paper on top of pastry and weigh down with second baking sheet to keep flat during baking. Bake for 15 to 18 minutes until golden brown and fully crisp. Cool completely on wire rack.
04 - In medium saucepan over medium heat, whisk together whole milk, heavy cream, granulated sugar, and cornstarch until completely smooth and combined.
05 - Whisk egg yolks in separate bowl. Gradually add small amount of hot milk mixture to yolks while whisking constantly to temper eggs. Pour warmed yolk mixture back into saucepan.
06 - Cook over medium heat, stirring constantly with whisk or silicone spatula, until mixture thickens considerably and just begins to bubble, approximately 5 minutes. Remove from heat immediately.
07 - Stir pure vanilla extract and softened unsalted butter into hot cream until butter melts completely and mixture is silky smooth. Transfer to clean bowl, press plastic wrap directly onto surface to prevent skin formation, and refrigerate until completely cold, at least 1 hour.
08 - Place one pastry rectangle on serving plate. Pipe or spread generous layer of chilled vanilla cream over pastry. Distribute fresh raspberries evenly over cream. Repeat with second pastry rectangle, another layer of cream, and more raspberries. Top with third pastry rectangle to create three-layer stack.
09 - Dust top of each Napoleon generously with powdered sugar using fine-mesh sieve. Garnish with fresh mint leaves if desired. Repeat assembly process to create 6 complete Napoleons. Serve immediately for optimal crispness or refrigerate up to 2 hours before serving.