Preheat oven to 350°.
Line a baking sheet with parchment paper. Scrape off the foam from the melted butter and pour the clear liquid into a small bowl, leaving the milk solids behind. Lay a piece of phyllo on the baking sheet, lightly brush with butter, top with a second sheet, brush with butter and sprinkle with sugar. Lay a third sheet on top, brush with butter and sprinkle with sugar. Cut phyllo layers, crosswise, into three equal rectangles. Top with a parchment sheet and cookie sheet. Bake 10–15 minutes, or until golden brown. Cool.
Hull the strawberries and slice lengthwise. Spread pastry cream on a piece of the baked phyllo. Top with about 1¼ cup strawberries.
Repeat with a second piece of phyllo and place on top of the first piece. Now add the remaining piece on top of the other two and dust with powdered sugar.
To serve, cut napoleon into 6 equal slices using a bread knife with a sawing motion. Place napoleons on serving plates and decorate with the remaining strawberries.
For the Pastry Cream:
Sift the sugar, flour and cornstarch together in a mixing bowl. Add the eggs and whisk until light.
Bring the milk to a boil in a heavy saucepan. Slowly whisk about half of the milk into the egg mixture, then pour the entire mixture back into the pan.
Cook over medium heat, stirring constantly, until the center bubbles and the mixture is thick. Remove from heat and stir in thebutter and vanilla. Strain through a fine-mesh strainer into a clean bowl. Cover with plastic and chill.