Prepare the Custard Cream:

1. Heat milk in a saucepan over medium heat until hot but not boiling.
2. In a bowl, whisk together egg yolks, sugar, vanilla, and flour until pale and smooth.
3. Gradually pour the warm milk into the egg mixture, whisking constantly.
4. Return to the saucepan and cook over medium heat, stirring continuously until thickened.
5. Transfer custard to a bowl, cover with plastic wrap (touching the surface), and chill for 1 hour.

2. Prepare the Pastry Horns:

1. Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
2. Sprinkle sugar on a work surface and roll out puff pastry into a 9×12-inch rectangle.
3. Cut into 12 strips, about 1 inch wide.
4. Lightly grease cream horn molds and wrap each strip around a mold, slightly overlapping.
5. Place seam-side down on the baking sheet, brush with egg wash.
6. Bake for 15–20 minutes, or until golden brown.
7. Let cool slightly before removing molds.

3. Assemble the Cannoncini:

1. Transfer chilled custard to a piping bag.
2. Pipe the custard into cooled pastry horns, filling generously.
3. Dust with powdered sugar before serving.