Pain au Chocolat

Place dough on a lightly floured work surface; cover and let stand until butter is slightly softened, 5 to 10 minutes.

Roll dough out to a 10-by-18-inch rectangle. Cut rectangle in half crosswise. Wrap one half with plastic wrap and transfer to refrigerator. Place remaining half on a large (about 18-by-25-inch) piece of parchment paper. Roll into a 16-by-20-inch rectangle, stopping to chill dough as necessary if butter becomes too soft; transfer to a large baking sheet and cover with plastic wrap. Transfer baking sheet to refrigerator and chill until firm, about 30 minutes. Repeat process with remaining piece of dough.

Transfer one piece of dough to a lightly floured work surface so that one of the longer sides is facing you. Halve dough to make two 16-by-10-inch rectangles.

Using a pizza cutter, trim a scant 1/8 inch from all sides of each rectangle of dough. Working quickly, using a pizza cutter, cut each half into seven 3-by-7-inch rectangles. Place 2 teaspoons chocolate in the center of each rectangle, leaving a 1/4-inch border all around. Starting from one short end and working toward the other, roll to enclose. Brush edges with water and gently press to adhere.

Line baking sheets with parchment paper; place 4 rectangles on each prepared baking sheet, seam-side down. In a small bowl, whisk egg with 1 teaspoon water. Brush tops and seams with egg mixture, reserving remaining egg mixture. Place 3 narrow drinking glasses upside-down between each roll of dough; cover well with plastic wrap, making sure plastic does not touch dough. Let stand in a warm place until dough has doubled in size and feels hollow when gently touched, 2 1/2 to 3 hours.

Preheat oven to 425 degrees.

Uncover and remove glasses from baking sheets; gently press down on each roll of dough and brush lightly with remaining egg mixture. Transfer to oven and bake until puffed and golden, about 15 minutes. Transfer to a wire rack to cool. Repeat steps 3 through 7 with remaining piece of dough.