Mix yeast, warm water, and sugar in a small bowl.
Let it sit until the mixture foams slightly.

Heat the milk until it’s lukewarm, then dissolve the salt and remaining sugar.
Place the flour in the stand mixer bowl with the dough hook attachment.
Add oil, yeast mixture, and milk mixture.
Mix all ingredients until all the flour is incorporated

Knead the dough for 5 minutes, until the mixture is smooth and homogeneous.
Place the dough in a bowl, cover it with plastic wrap.
Let it rise for 3 hours at room temperature, or until it has tripled in volume.

Folding:
Roll out the dough on a floured surface into a large rectangle of 14 x 8 inches (35 cm x 20 cm).
Spread the butter over 2/3 of the dough and fold it in 3 (the unbuttered part in the middle, then the last 1/3 on top).
Refrigerate for 30 minutes.
Turn the dough 1/4 turn and roll out again.
Fold it in 3 and refrigerate again for 30 minutes.
Repeat the operation 3 times.
After the final turn, refrigerate the dough, wrapped in plastic wrap, for 5 hours or overnight.

Shaping:
On a floured surface, roll out the dough into a rectangle of 15 x 6 inches (38 cm x 15 cm).
Using a pizza wheel, cut out triangles of 3 inches (7.5 cm) base.
Place chocolate bars on the base, if making chocolate croissants, then roll the dough while stretching it.

Place the croissants 2 inches (5 cm) apart on baking sheets lined with parchment paper.
Cover them with plastic wrap and let them rise for 1 hour in a warm place (72°F/22°C) or until they have doubled in volume.

Baking:
Preheat the oven to 475°F (240°C).
Mix the egg with a tablespoon of water, then brush the croissants with a brush.
Bake the croissants in the oven for 12 to 15 minutes, until the top is golden brown.
Remove the croissants from the oven and place them on a rack.
Let cool for 10 minutes before serving.