As the dough has to rise overnight in your fridge, start with your dough in the early evening.Warm your milk in a microwave or a pot over your cooking stove. Do NOT cook the milk, just let it warm up.
Put the yeast with 5g of the sugar and two tablespoons of the warm milk in a small bowl and let it sit for a few minutes until foaming.
Meanwhile, mix the rest of the sugar, flour, and salt in your kitchen machine. Add the foaming yeast and knead it until you have a well-combined dough.
Cover with a damp kitchen towel and let it rise at room temperature for one hour.
Place your dough on a well-floured working surface and roll it out into a large rectangle. Only roll in one direction.
Cut the cold butter into thin pieces and place half of them onto your dough. I prefer to place the butter in the middle of the rectangle and leave about 2cm on each side without butter. Otherwise, it can get a bit messy if you roll it out again.
Fold the dough from the top down three times.
Repeat step number 6 with the other half of the butter. Fold the dough again from the top down three times.
Cover your dough in a plastic wrap and leave it in your fridge overnight.
The next morning, place your dough on a well-floured working surface and roll it out into a rectangle which has about the size of 45 cm x 20 cm. Your dough should be about 1cm thick. Cut away the irregular parts.
Cut into 8 rectangles.
Pain au Chocolat
Chop the dark chocolate lengthwise and place it on the short end of your rectangles.
Roll them up into rolls and place them on a with baking paper lined baking sheet. As your pain au chocolat will rise in the oven, it's better to divide them between two baking sheets.
Let them rise for one hour in your fridge if possible. If you don't have enough space in your fridge, let them rise for one hour at room temperature.
Preheat your oven to 220°C/430°F (conventional).
Mix the egg with milk and brush your pain au chocolat with it.Bake them for 15 minutes in your preheated oven. It's normal that some butter will flow out during baking. Just make sure to use a large enough baking paper to prevent cleaning the burned butter from your oven and baking tray.Bake one sheet after the other.
Let them cool down on a cooling rack before enjoying them still slightly warm.
I don't like to throw the trimmed dough ends away and shape them instead into little croissants.
Use milk chocolate or chocolate spread instead of dark chocolate.