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 measuring approximately 2.5 x 4 inches. Transfer to prepared baking sheet and prick each piece several times with fork to prevent excessive rising.
03 - Place another sheet of parchment paper on top of pastry rectangles, then weigh down with a second baking sheet to keep pastry flat during baking. Bake for 15 to 18 minutes until golden brown and crisp. Cool completely on wire rack.
04 - In medium saucepan, whisk together whole milk, heavy cream, granulated sugar, and cornstarch until smooth and no lumps remain.
05 - In separate bowl, whisk egg yolks until pale and slightly thickened. Gradually whisk approximately 1/2 cup of hot milk mixture into yolks to temper, then pour warmed yolk mixture back into saucepan with remaining milk mixture.
06 - Cook over medium heat, stirring constantly with whisk or heat-resistant spatula, until mixture thickens considerably and just begins to bubble, about 5 minutes. Remove from heat immediately.
07 - Stir vanilla extract and softened butter into hot cream until butter melts completely and mixture becomes smooth and glossy. Transfer to clean bowl, press plastic wrap directly onto surface to prevent skin formation, and refrigerate until completely chilled, at least 1 hour.
08 - Place one pastry rectangle on serving plate. Pipe or spread layer of chilled vanilla cream over pastry. Distribute fresh raspberries evenly over cream layer. Repeat with second pastry rectangle, another layer of cream, and additional raspberries. Top with third pastry rectangle.
09 - Dust tops generously with powdered sugar using fine-mesh sieve. Garnish with fresh mint leaves if desired. Repeat assembly process to create 6 total Napoleons. Serve immediately for optimal crispness, or refrigerate up to 2 hours before serving.