01 - Preheat oven to 400°F. Line a baking sheet with parchment paper and set aside.
02 - Roll out thawed puff pastry on a lightly floured surface to smooth any 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 to prevent puffing.
03 - Place a second sheet of parchment paper on top of the pastry pieces, then weigh down with another baking sheet to keep layers flat during baking. Bake for 15 to 18 minutes until golden brown and crisp. Cool completely on a wire rack before assembling.
04 - In a 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 a separate bowl. Gradually add a small amount of the hot milk mixture to the yolks while whisking constantly to temper. Return the mixture to the saucepan.
06 - Cook over medium heat, stirring constantly with a heatproof spatula or whisk, until mixture thickens considerably and just begins to bubble, approximately 5 minutes. Remove from heat immediately.
07 - Stir in vanilla extract and softened butter until completely incorporated and smooth. Transfer to a clean bowl and press plastic wrap directly onto the surface to prevent skin formation. Refrigerate until completely chilled, at least 1 hour.
08 - Place one pastry rectangle on a serving plate. Pipe or spread a generous layer of chilled vanilla cream over the surface, then distribute fresh raspberries evenly. Repeat with a second pastry layer, more cream, and additional raspberries. Crown with a third pastry rectangle.
09 - Dust the tops generously with powdered sugar using a fine-mesh sieve. Garnish with fresh mint leaves if desired. Repeat assembly process to create 6 complete Napoleons. Serve immediately for maximum crispness, or refrigerate for up to 2 hours before serving.