Begin by make custard cream:
Heat the milk until it is hot (not boiling).
In a medium saucepan, whisk together egg yolks, sugar, vanilla extract, and flour until light and fluffy.
Add a little milk at a time while whisking, making sure there are no lumps.

Place the skillet over medium heat and stir constantly until it comes to a slow boil. The cream will thick, so be careful not to letting it stick to bottom. Lower the flame and let it cook for a few more minutes, until it reaches the desired intensity.
Pour the cream into a glass container, cover with plastic wrap and leave to cool. Refrigerate about at least 1 h.
For pastry horns:
Preheat the oven to 400°F (200°C).
Sprinkle some sugar on the counter and over the puff pastry and roll it into a rectangle about 9 x 12 inches.
Cut into 12 strips (about 1 inch thick). An easy way to do this: cut the dough into three parts, and then each part into four slices.

Roll each strip onto a horn template (cone shaped). The pastry should overlap (about half the length).
Lay it out on a baking sheet lined with parchment paper with the side (the end of the strip) facing down.
Beat 1 egg with a tsp water. Gently brush each piece of pastry with the beaten egg.
Make sure to keep eggs away from mold. This will make it difficult to remove the pastry horn from the mold once it’s baked.
Bake at 400 F (200 C) for 15-20 minutes until golden on top.
Let them cool for a few minutes and then gently remove them from the mold. If the dough sticks to the mold, you can press down on the mold a little (to make the circumference smaller) and gently flip it inside the pastry until it pops.
Before serving, fill with cream using a piping bag. Sprinkling with powder sugar if you like, & Enjoy!