4 oz. bar bittersweet chocolate (or approximately 1 1/2 cups bittersweet chocolate chips)
Prepare dough as directed in linked recipe through step 7.
To make the croissants: Halve the dough; refrigerate one half. Roll dough out to a 16 x 12" rectangle, trimming the edges and corners to square.
With the short side facing you, cut in half horizontally and return one half to the refrigerator. Cut remaining dough portion vertically into thirds (rectangles), then cut diagonally into triangles.
Line two baking sheets with parchment paper or silicone mats. Cut chocolate into ⅓" wide pieces that are about the width of the wide end of your dough triangles.
Working with one triangle at a time, stretch the dough by about 50%. Place chocolate at the end, then roll from the wide end to the pointed tip. Place on baking sheet so tip is on the bottom, and slightly curve them into the classic croissant shape. Repeat with remaining triangles and dough.
Allow to rise for 2 hours. Preheat oven to 425F; position racks in the top and bottom thirds of the oven. Brush pastries with beaten egg.
To bake: Spritz the inside of a hot oven with water; close door. Place pastries into oven, spritz oven again with water, close door and reduce heat to 400F. Bake for 10 minutes. Rotate pans, lower oven temperature to 375 and bake for an additional 10 minutes. Remove from oven and allow to cool slightly before serving.