Use cooking spray to spritz the inside of a gallon sized zipper top bag. Turn the dough out onto the counter, flatten it gently with the palms of your hands, and fold in thirds like a letter. Slide the dough into the prepared bag, zip the bag shut, and place into the refrigerator for at least 2 hours, but up to 24 hours.

While the dough is chilling, prepare your butter. This can be done as soon as you've placed the dough in the refrigerator, if you'd like. Cut 2 half-cup sticks of butter in half lengthwise. This will give you four long rectangles of butter. Set aside. Lay a 12-inch or longer piece of waxed paper or plastic wrap. Sprinkle thoroughly with flour. Lay the sticks of butter side by side (touching), sprinkle with more flour, and lay another piece of waxed paper or plastic wrap over the top. Gently smack with a rolling pin until the butter becomes malleable, then roll it out until it is approximately 6-inches by 9-inches. Wrap well with the waxed paper or plastic wrap with which you rolled it and return it to the refrigerator or at least an hour or until you're ready to deal with the dough.

Take the refrigerated dough from the bag and place onto your counter. Roll out until it forms a rectangle that is approximately 10-inches by 20-inches. Take the refrigerated butter from its wrapper and center the butter on top of the dough. This will leave you with about 1/3 of the dough open, followed by 1/3 covered by butter, followed by another open 1/3 of dough. Fold an open side of dough over the butter, then fold the other open side of dough over the previously folded side of dough, much like you're folding a letter into thirds. Now that the dough is folded, pinch the edges together thoroughly, sealing the butter into the dough completely. Roll the dough out into a 10-inch by 24-inch rectangle. This time, fold both ends in toward the center to meet in the middle, then fold that in half like closing a book. Wrap this up with floured plastic wrap and return the whole thing to the refrigerator for at least 2 hours but up to 7 days.

Remove the dough from the refrigerator, unwrap, cut in half, re-wrap half and return it to the refrigerator. Roll out the remaining half of the dough into a rectangle that is roughly 9-inches by 25-inches. Use a pizza cutter, bench knife, or sharp paring knife to trim the sides neatly to an 8-inch by 24-inch rectangle. Cut the dough in half lengthwise, then crosswise. Cut the long rectangle quarters into half again, yielding 8 rectangles that are about 4-inches by 6 inches each.

Lay 2 to 3 chocolate batons near the end of each rectangle and roll the dough around them. Lay the tube, seam side down, on a parchment or silpat lined half-sheet pan, leaving at least 2 to 3 inches of free space all the way around. Gently press the tube to flatten. A half sheet pan should comfortably fit 8 unbaked pain au chocolat. Lay a clean tea towel over the pan and let rest at room temperature for 30 minutes, or until slightly puffy.

Preheat the oven to 425F. Whisk together a large egg with a pinch of salt until even in colour. Brush the tops of the pain au chocolat with the egg wash. Bake for 15 to 18 minutes, or until the pastry is puffed, set, flaky, and caramelized to a deep rosewood brown. Let the pastries rest on the pan for 5 minutes before transferring to a cooling rack. These are always best when eaten slightly warm on the day they were made, but if there is some weird circumstance in which they're not all eaten immediately, I have loved them the day after they were made, too.