Published in the 2015 holiday issue of Bon Appétit magazine and now in Claire Saffitz’s cookbook, Dessert Person, this recipe combines babka with challah in one tasty loaf. Although traditional challah isn't made with dairy, Claire uses milk and butter in the loaf itself as well as butter in the cinnamon-chocolate filling. Once you've tried this version of the recipe, you can treat it as a jumping-off point for a host of other flavorful fillings: jam, chocolate-hazelnut spread, cookie butter...
To make the dough: In a large bowl, stir together the milk, yeast, sugar, egg yolks, and vanilla. Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess. Add the flour, salt, and butter and mix with a sturdy spoon or your hands to form a shaggy dough.
Turn the dough out onto a lightly floured surface and knead, adding a bit more flour as necessary to prevent sticking, until the dough is smooth, supple, and no longer shiny, 8 to 10 minutes. Alternatively, you can knead using your stand mixer fitted with the dough hook attachment, mixing on medium speed for 5 to 8 minutes.
Lightly butter the inside of a large clean bowl, then gather the dough into a ball and place it in the bowl. Cover the bowl and let the dough rise in a warm place until it’s puffy, 1 1/2 to 2 1/2 hours, depending on the temperature of your kitchen.
To make the filling: In a small bowl, toss together the chocolate, brown sugar, cinnamon, and salt; set aside.
To assemble the babkallah: Turn the dough out onto a clean work surface and divide it into three equal portions, about 250g each. Shape each portion into a 12” rope.
Flatten each rope with the heel of your hand, then use a rolling pin to roll each piece into a 12” × 6" rectangle (you shouldn't need any flour to prevent sticking).
Brush the surface of each rectangle with the melted butter and sprinkle with the chocolate mixture, dividing it evenly among the three pieces and leaving a 1/2" border uncovered along one long edge of each piece of dough.
Starting on the filling-covered long side, roll each rectangle into a spiraled log and pinch along the length of the seams to seal; it's important to seal the logs very well so they don’t unravel as you braid, which could compromise the spiral of filling in the finished babkallah.
Place the logs seam-side down and side by side on a parchment-lined baking sheet. Pinch the logs together at one end and braid them, leaving a little slack in the braid as you go to allow for expansion.
Pinch the opposite ends of the braid together and tuck both ends underneath the braid.
Cover the babkallah loosely and set it aside in a warm place until it’s expanded to about 1 1/2 times its original size, 1 to 2 hours.
Toward the end of the rise time, place an oven rack in the center of the oven and preheat the oven to 350°F.
Brush the babkallah with the beaten egg, then sprinkle generously with Demerara sugar.
To bake the babkallah: Bake the babkallah until the crust is deeply browned, 35 to 45 minutes; a digital thermometer inserted into the center will read 190°F. Remove it from the oven and transfer it to a rack to cool completely.
The babkallah, well wrapped and stored at room temperature, will keep up to four days, but it's best served on the first or second day.