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 quarters lengthwise, then once crosswise, yielding 8 long rectangles.
Pipe about 3 tablespoons of pastry cream onto half of each rectangle, leaving an empty 1/2-inch of border pastry. Sprinkle a well-rounded tablespoon of the miniature chocolate chips over the pastry cream. Fold the empty half of pastry up over the filled half. Gently press the pastry together at the end. Transfer to a parchment or silpat lined, rimmed baking sheet.
A half sheet pan should comfortably fit 8 unbaked plie 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 plie au chocolat with the egg wash. Bake for 23 to 30 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.