Warm up the milk until hot (not boiling).

In a medium-sized pan, whisk together the egg yolks, sugar, vanilla extract, and flour until light and fluffy.

Gradually add the hot milk to the egg mixture while whisking, ensuring there are no lumps.

Set the pan over medium heat and stir continuously until the mixture reaches a slow boil. Continue cooking for a couple of minutes until it thickens to the desired consistency.

Pour the custard cream into a glass bowl, cover with plastic wrap, and let it cool down. Chill for at least one hour.

Preheat the oven to 400°F (200°C).

Sprinkle some sugar on the counter and on top of the puff pastry. Roll out the puff pastry into a rectangle about 9 by 12 inches.

Cut the pastry into 12 strips, each about 1 inch thick. You can easily achieve this by dividing the pastry into three parts and then cutting each part into four strips.

Roll each strip onto a horn mold, ensuring the pastry overlaps by about half of its length.

Place the rolled pastry horns on a baking sheet lined with parchment paper, with the seam side down.

In a small bowl, beat the egg with one tablespoon of water to make an egg wash. Lightly brush each pastry cone with the egg wash, avoiding the mold.

Bake the pastry horns in the preheated oven for about 15-20 minutes or until they turn golden brown on top.

Allow the pastry horns to cool for a couple of minutes before gently removing them from the molds.

Sprinkle the cooled pastry horns with powdered sugar for decoration before serving. Enjoy your delightful puff pastry horns filled with custard cream!