I used one sheet and made 6. The original recipe from https://www.bonappetit.com/story/choc-bar-puffs used two sheets and make 15.
1 sheet frozen puff pastry
About 2 thin chocolate bars - nice quality
1 egg
Flake salt and/or sugar to sprinkle
Preheat oven to 400 F.
Lay a sheet of parchment on the counter and roll out to a rectangle a little bigger than 12”x9”. Cut in half so you have two pieces 6”x9”. Break up your chocolate so you have 6 pieces that fit on one piece with at least 1/2” pastry border on all outside edges and at least 1” between chocolate pieces. (These borders will be sealed so the chocolately goodness doesn’t escape.). I used my favorite 85% chocolate bar... it was a *little* bitter for this application. Next time I’ll probably add a sprinkle of sugar on top of the bar before putting the upper pastry on.
Whisk the egg in a small dish and paint egg wash all around the chocolate pieces. Lift the other piece of pastry and set on top. Press the pastry together around all edges, trying to get any air bubbles out. Brush egg wash over the top and cut into 6 croissants. Separate slightly and use parchment as a sling to transfer to baking sheet. Sprinkle top with flake salt and/or sugar.
If at any point you have to wait on the oven (or it’s hot in your kitchen), pop the pastry back in the fridge to keep cool.
Bake until the tops are deep gold and taller than you thought they could get. Original recipe said 20-25 minutes, rotating halfway through. I have much better results preheating to 400 F, turning down to 385 F when I put pastries in the oven and baking for just 16 minutes.