How to Make It
Line 2 baking sheets with parchment paper; set aside. Uncover refrigerated croissant dough, and transfer to a lightly floured surface. Lightly flour top of dough; roll into a 19- x 11-inch rectangle. Using a pizza cutter, trim edges to form an 18- x 10-inch rectangle; discard scraps. Cut dough into 12 (5- x 3-inch) rectangles. Place rectangles in a single layer on 1 prepared pan; cover tightly with plastic wrap, and refrigerate 15 minutes.
Place 1 dough rectangle on a clean work surface with a short side closest to you. Place 1 chocolate baking stick crosswise, 1/2 inch from bottom short side of dough rectangle. Place a second chocolate baking stick crosswise, 1 1/2 inches from top short side. Starting at bottom short side, roll dough over first chocolate baking stick, and continue rolling over second chocolate baking stick and to top edge of dough. Firmly press seam to secure (chocolate baking sticks should be side by side, like binoculars, with a layer of dough in between). Place rolled dough, seam side down, on prepared baking sheet. Repeat with remaining dough rectangles and chocolate baking sticks, placing pastries at least 2 inches apart on both baking sheets. Proceed with recipe, or cover with plastic wrap and refrigerate up to 8 hours or overnight for deeper flavor.
Whisk together egg and milk in a small bowl. Brush pastries lightly with egg mixture, using a paper towel to wipe away excess. Reserve remaining egg mixture in refrigerator. To proof pastries, add 2 cups hot water to a small bowl, and place in a cold oven. Place baking sheets with pastries, uncovered, in oven with hot water. Close door, and proof until pastries double in size and jiggle when you shake the baking sheets, 1 to 2 hours. Remove pastries and water from oven, and let pastries stand at room temperature 30 minutes. Meanwhile, preheat oven to 425°F. Place oven rack in middle position.
Gently brush 1 baking sheet of pastries with a second coat of egg mixture. Place baking sheet on middle rack in preheated oven, and immediately decrease temperature to 375°F. Bake until golden brown and crisp, 18 to 24 minutes. Transfer pain au chocolat to a wire rack. Return oven temperature to 425°F. Repeat process with egg mixture and remaining pastries. Serve warm, or let cool completely, about 1 hour. (Pain au chocolat are best eaten the day they are baked.)
Like croissants, pain au chocolat bake the most evenly on the middle oven rack. If your oven is large enough to accommodate 2 pans side by side, both batches may be baked at once.