For the custard: Heat milk in a medium saucepan until hot but not boiling (about 180°F/82°C). Meanwhile, in a separate bowl, vigorously whisk egg yolks with sugar and vanilla until pale yellow and fluffy (about 2 minutes). Add flour and whisk until completely smooth with no lumps. Gradually pour about ¼ cup of the hot milk into the egg mixture while whisking continuously to temper the eggs. Then slowly pour the egg mixture back into the remaining hot milk in the saucepan, whisking constantly.

Cook the custard over medium heat, stirring continuously with a wooden spoon or heat-resistant spatula, making sure to scrape the bottom and sides of the pan. Continue cooking until the mixture thickens and coats the back of a spoon (about 5-7 minutes). Remove from heat, strain through a fine-mesh sieve if there are any lumps, and transfer to a bowl. Press plastic wrap directly onto the surface of the custard to prevent a skin from forming. Refrigerate until completely cooled (at least 2 hours or overnight).

For the pastry: Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper. Lightly flour your work surface. Gently unfold the defrosted puff pastry sheet and roll it slightly to smooth out any creases. Using a sharp knife or pizza cutter, cut the pastry sheet into 12 equal strips, approximately 1 inch (2.5cm) wide and 8 inches (20cm) long.

Sprinkle each strip generously with granulated sugar on both sides, pressing slightly to adhere. Starting at the pointed end of a cream horn mold, wrap each strip around it in a spiral pattern, overlapping the pastry slightly as you go (about ¼ inch/0.5cm overlap). Make sure the sugar-coated side faces outward. Place seam-side down on the prepared baking sheet, leaving at least 1 inch (2.5cm) between each pastry to allow for expansion.

In a small bowl, beat the egg with 1 tablespoon water to create an egg wash. Brush each pastry horn thoroughly with the egg wash, being careful not to let it pool at the base. Bake in the preheated oven for 20-25 minutes, or until deeply golden brown and puffed. Check at 15 minutes - if they're browning too quickly, reduce temperature to 375°F (190°C).

Once baked, let the pastries cool on the molds for 5 minutes, then carefully remove each horn from its mold while still warm (they'll harden as they cool). Place on a wire rack to cool completely (about 30 minutes).

When ready to serve, remove the chilled custard from the refrigerator and whisk until smooth. Transfer to a piping bag fitted with a star tip (½-inch/1cm opening). Carefully pipe the custard into each pastry horn, filling from both ends to ensure the center is filled. Dust generously with powdered sugar using a fine-mesh sieve. Serve immediately or refrigerate for up to 2 hours before serving (the pastry will soften slightly if stored too long).