How to Make It
In a medium saucepan, heat the milk until hot to the touch. Meanwhile, in a medium heatproof bowl, whisk the whole eggs with the egg yolks and sugar. Whisk in the flour, cornstarch and vanilla until smooth. Slowly whisk in the hot milk until incorporated. Pour the mixture into the saucepan and cook over moderate heat, whisking constantly, until bubbles appear around the edges and the custard is hot and thick, about 4 minutes. Pass the custard through a fine strainer into a bowl and refrigerate until chilled, stirring occasionally, about 20 minutes.
In a large stainless steel bowl, using an electric mixer, beat the heavy cream until stiff. Beat in the 1/4 cup of confectioners' sugar until combined. Fold the custard into the whipped cream. Cover with plastic wrap and refrigerate.
Preheat the oven to 350°. Line 2 large rimmed baking sheets with parchment paper. Lay 1 sheet of phyllo dough on a work surface and brush with melted butter. Top with another phyllo sheet and brush with butter. Repeat twice more so you have 4 layers. Cut out nine 4-inch squares. Cut each square in half diagonally so you have 18 triangles. Transfer the triangles to the lined baking sheets and cover with another sheet of parchment paper. Top with another baking sheet and bake for about 20 minutes, until the triangles are nicely browned. Repeat with the remaining phyllo sheets and butter in 2 batches, making another 36 triangles.
Place a phyllo triangle on each of 8 plates. Lightly spread with 1/2 tablespoon of dulce de leche and 1 tablespoon of the custard cream; top with another phyllo triangle. Repeat 4 more times, finishing with a sixth phyllo triangle. Top each napoleon with a sliced strawberry. Dust with confectioners' sugar and serve.