While the dough is chilling, prepare the filling. In a medium bowl combine the chopped chocolate, butter and cocoa. Microwave on medium (50%) power for 1 minute. Stir then repeat for 15 or 30-second intervals until the chocolate is melted and smooth. If little chunks of chocolate remain, set the mixture aside for a minute, stir, then repeat until smooth. You don’t want to over-cook the chocolate in the microwave. Add the powdered sugar and blend until smooth. Set aside to cool completely. Once cool, add 1 egg white and blend with a fork until fully combined. Remove 1 tablespoon of the chocolate mixture and set aside.
Grease an 8 ½ by 4 ½-inch loaf pan and set aside.
Lightly dust a clean work surface with flour. Gently press down the dough with your hand to deflate. Turn the dough out onto the prepared surface. Do not knead, fold or turn the dough, but gently press or roll the dough into an 18 by 14-inch rectangle, with the 18-inch side parallel to the edge of the counter. TIP: Roll the dough in such a way that the final ⅓ section farthest away is slightly thicker. The last ⅓ of the dough will be wrapped on the outside of the cylinder and this helps prevent splitting and leaking.
Using an off-set spatula, spread the filling (all but the reserved 1 tablespoon) over the dough leaving a ½-inch border around the edges.
Pushing away from you, roll the dough into a cylinder, keeping the roll tight by tucking it under itself as you go. TIP: I find a lightly floured bench scraper aids tremendously in this process. Pinch the seams together and reshape as needed to get an 18-inch cylinder of uniform thickness. With the seam side up, spread the remaining 1 tablespoon of filling over the seam in a thin (1-inch) line. Fold the cylinder in half bringing it on top of itself. Pinch the ends together to seal.
Gently twist the cylinder twice to form a double figure eight. Using the bench scraper to help, lift the loaf into the prepared pan. Gently Press the dough into the corners of the pan. Lightly cover the loaf in plastic wrap and let rise until the dough is level with the lip of the pan, about 1 ½ to 2 hours. Don’t overproof the dough!
When ready to bake, adjust the oven rack to the lower-middle position and heat to 350°F. Gently brush the loaf with the egg wash mixture and bake for 40 to 45 minutes or until it registers 190°F to 195°F degrees on an instant read thermometer. The loaf will be a deep golden brown. Cool the bread in the pan for 15 minutes, then remove to a wire rack to cool completely, about 3 hours.