Begin by heating the milk in a saucepan until it’s hot (but not boiling). In a separate medium-sized pan, whisk together the egg yolks, sugar, vanilla extract, and flour until the mixture becomes light and fluffy. Gradually add the hot milk, stirring continuously to avoid any lumps. Place the pan over medium heat and stir constantly until the mixture begins to gently boil. As it heats, the cream will thicken, so be cautious to prevent it from sticking to the bottom. Lower the heat and continue cooking for another couple of minutes until it reaches your desired thickness. Transfer the cream to a glass bowl, cover with plastic wrap, and let it cool. Once cooled, refrigerate for at least one hour.

Preheat the oven to 400°F (200°C). Lightly sprinkle some sugar on your countertop and on top of the puff pastry, then roll it out into a rectangle roughly 9 by 12 inches. Cut the pastry into 12 strips, each about 1 inch wide. An easy way to do this is to divide the pastry into three parts and then cut each part into four strips. Take each strip and wrap it around a conical-shaped horn mold, making sure the pastry overlaps by about half its length. Place the wrapped pastry cones on a baking sheet lined with parchment paper, ensuring the seam (the end of the strip) is facing down. Beat an egg with a tablespoon of water and lightly brush the egg wash over each pastry cone. Avoid getting egg wash on the mold itself, as this could make it difficult to remove the pastry after baking. Bake at 400°F (200°C) for about 15-20 minutes, or until golden on top. Once baked, let them cool for a couple of minutes before gently removing the pastry from the molds. If the pastry sticks, gently press the mold to make it slightly smaller, and then carefully twist it until the pastry releases.

Before serving, pipe the chilled custard cream into each pastry horn. If desired, sprinkle with powdered sugar and enjoy!