Torsades au Chocolat ~ Chocolate Pastry Twists

In a medium saucepan over medium heat, bring the milk to a boil.
Meanwhile, in a separate medium saucepan, whisk the egg yolk, sugar, and cornstarch together.
Once the milk has just reached a boil, pour a couple of teaspoons of the hot milk into the egg yolk mixture and whisk until combined.
Slowly pour in the rest of the hot milk, whisking constantly.
Set the saucepan over medium heat, continuing to whisk constantly. Cook until the mixture starts to thicken slightly and produce slow large bubbles, 1 to 3 minutes. Remove from the heat. If your mixture has overcooked a little bit and you have bits of cooked egg in your custard, strain the custard into a clean bowl using a fine-mesh sieve.
Whisk in the vanilla, then the butter, until completely incorporated.
Scrape the custard into a clean bowl, cover it with plastic wrap (the wrap should be touching the surface of the custard to prevent a skin from forming), and allow the custard to come to room temperature. If not using immediately, refrigerate until you are ready to use. Before you use it, you might have to give the custard a whisk to loosen it.

Preheat the oven to 375˚F (190°C) and have a rimmed baking sheet ready.
Roll your pastry out on parchment paper until it’s approximately 12-by-12 inches (30-by-30 cm).
Spread the pastry cream over the entire surface of the pastry.
Sprinkle the chocolate over one half of the pastry and fold the other half of the pastry to cover it.
With the open end of the pastry facing you (you’ll have a rectangle of 12-by-6 inches [30-by-15 cm]), cut strips of the pastry approximately 2 inches (5 cm) wide.
Lift the parchment onto a rimmed baking sheet and separate the pastry strips slightly.
Twist the pastry strips in one direction at one end and in the other direction at the other end to make a twist. Make two twists at each end. Some of the chocolate might fall out–but just press it back into the pastry cream. Make sure your twists have a lot of room between them on the baking sheet.
Lightly brush the twists with the beaten egg, avoiding the pastry cream and chocolate edge of the pastry.
Bake until the pastry is puffed and golden brown, 25 to 30 minutes.
Remove the twists from the oven, leaving them on the baking sheet.
In a small heatproof bowl, stir the boiling water and sugar together and brush the still hot twists with the sugar water. This will dry as they cool and will make them lovely and shiny.