Babka au Chocolat Brioche

Directions

1. Spread the flour in a shallow baking pan. Cover and refrigerate or freeze until needed, at least 45 minutes.
2. Use the paddle attachment to whip the cold butter only until it is smooth and slightly fluffy, but still cold. There should be no small hard lumps when you pinch it between your fingers. Scrape the butter into a mound on a piece of wax paper and refrigerate it until needed. Wash the mixer bowl. (Proceed with the recipe right away; too long a delay will re-harden the butter.)
3. Dissolve the yeast and 1 teaspoon of sugar in the water in the bottom of the clean mixer bowl, now fitted with the dough hook. Add the remaining 1/3 cup (66 grams) sugar, eggs, yogurt, salt, and flour. Mix on low speed until the ingredients are blended, scraping the bowl as necessary. Turn the speed up to medium (speed 5) and knead for 5 minutes. At the end of the kneading period the dough will be soft, moist, and sticky, and very elastic. All or most of it will be wrapped around the dough hook. Add the cold creamed butter in several large pieces, pushing it into the dough, and beat with the hook until it is well blended. You will need to stop the mixer several times to scrape the dough from the sides of the bowl and pull it completely off of the hook, until the butter appears thoroughly incorporated. Scrape the dough into a bowl, cover, and refrigerate overnight, or up to 24 hours.
4. Thoroughly mix the brown sugar, cocoa powder, espresso powder, cinnamon, and salt. Cover and set aside. Grease an 8 to 10 cup tube pan with a removable bottom.
5. Scrape the cold dough out onto a well-floured surface. Use a rolling pin to roll the dough into a thin square sheet about 18" by 18", sliding the dough to be sure it’s not sticking and re-dusting the surface beneath it as necessary. Mix the cocoa mixture with 2 teaspoons water—it should resemble slightly damp sand. Scatter and spread it evenly over the dough, leaving a 1-inch margin on the edges nearest and furthest from you. Scatter the chopped chocolate evenly over the cocoa mixture. Moisten the far edge of the dough with water. Beginning at the closest edge, roll the dough and filling into a tight jelly roll. Press firmly to seal the dough. Seam side-down, cut the roll into eighteen 1" slices with a sharp knife. Place the slices gently in pan haphazardly, without particularly arranging them. If you lay them flat in the pan, they will not stick together properly. Adjust the slices to reach the same level in the pan. Cover the pan loosely with plastic wrap and let rise in a warm place until almost doubled, about 2 hours.
6. Preheat the oven to 350°F. Brush the egg wash gently over the surface of the babka (a natural bristle brush does the best job of this!). (Tip: you will have most of the egg wash left over, refrigerate it and add it to your breakfast scramble). Place the pan on a baking sheet. Bake for 50-60 minutes, or until the top is deep browned and the bottom of the pan sounds hollow when tapped, or until an instant read thermometer register 190°F when inserted in the center of the bread. Cool in pan on a rack for 10 minutes, then pull the tube up to detach the babka from the sides of the pan before it has a chance to cool and stick—and slide a knife or thin metal spatula between the babka and the bottom of the pan.