Bring milk to boiling in medium-size saucepan over medium heat.
Stir together cornstarch, granulated sugar and salt in small bowl. Stir in egg. Pour a little milk into egg-cornstarch mixture and stir; then stir egg-cornstarch mixture into milk in saucepan. Cook over medium heat until thickened, 6 to 8 minutes. Stir in vanilla. Strain, if necessary, to remove any lumps. Cool. Cover surface of pastry cream with plastic wrap; refrigerate.
Heat oven to 350°F.
Meanwhile, prepare phyllo. Line baking sheet with parchment paper; place 1 sheet of phyllo on top of parchment. Lightly coat phyllo sheet with nonstick cooking spray; sprinkle evenly with half of the confectioners sugar. Place second layer of phyllo on top; lightly coat with cooking spray again; sprinkle with remaining confectioners sugar. Top with final sheet of phyllo. Carefully cut into 12 equal rectangles. Cover with another sheet of parchment paper. Place a baking sheet on top.
Bake pastry in 350°F oven for 12 to 15 minutes. Remove pan to wire rack to cool.
Place one 3-layered rectangle of phyllo on dessert plate. Spread top with 1-1/2 tablespoons pastry cream. Top with a few raspberries, blueberries and 1 sliced strawberry. Repeat layering with another phyllo rectangle, pastry cream and fruit; for each Napoleon use a total of 1/4 cup raspberries, 1/4 cup blueberries and 2 sliced strawberries. Top with a third rectangle of phyllo. Garnish with a raspberry and mint. Repeat with remaining phyllo and berries to make a total 4 Napoleons.