Preheat the oven to 375.
Unfold the puff pastry. Using a sharp knife or pizza cutter, cut along the fold lines, then cut into thirds the opposite direction, so you end up with 9 squares.
Spray a cookie sheet or line with parchment paper. Place each square on the sheet, spacing about 1" apart. Bake until puffed and golden 12-15 minutes. Remove from oven and allow to cool.
In a medium mixing bowl, whisk together the eggs, sugar, vanilla and flour. Pour the milk into a small saucepan and bring it up to a simmer.
Slowly add a small amount of the hot milk (a quarter of it) to the egg mixture while whisking constantly. Return the saucepan to the stove and pour the tempered egg mixture into the saucepan. Cook over medium-low heat, stirring until the mixture has thickened into a pudding-like consistency. Remove from the pan and allow to cool.
Use a serrated to cut each puff pastry square crosswise (so each thick square becomes two thin squares). Pour the filling into a pastry bag and cut the tip off.
Assemble the napoleons as follows: pastry, filling, sliced berries, small amount of filling to hold the next pastry in place. Repeat until you have 4 pastry layers and 3 berry layers. Dust with powdered sugar and serve.