Chocolate Babka

Make the dough: In a medium bowl, whisk together flour, granulated sugar, and salt. Set aside. In the mixing bowl of a stand mixer fitted with the dough hook attachment, add 2/3 cup (160 milliliters) water and yeast. Let stand until foamy, about 5 minutes. Add egg to the yeast mixture, and beat on low speed until blended, about 1 minute. Gradually add the flour mixture. Increase the speed to medium-low. Scraping the bowl as needed, beat until the dough is blended, 1–2 minutes. Reduce the speed to low and gradually add butter cubes to the dough, beat until the mixture is sticky, about 3 minutes.

Grease a large bowl and add the dough, turning to coat. Cover the bowl with plastic wrap and let the dough rise in a warm, draft-free place for 1½–2 hours.

Make the filling: In a small microwave-safe bowl, heat chocolate and butter on low power, stirring occasionally until the chocolate becomes slightly shiny and smooth. Add granulated sugar and espresso instant coffee; stir until well blended. The mixture will look grainy.

Divide the dough in half and cover one dough half with plastic wrap. On a floured surface, roll one dough half into a 10-by-12-inch (25-by-30-centimeter) rectangle. Leaving a 1-inch (3-centimeter) border around the dough, spread half of the filling over the dough. Starting at one short end, roll the dough to form a log. Pinch the seam to seal. Repeat with the remaining dough and filling. Twist each log several turns. Twist the two logs together, pinch the ends together, and fold the ends under. Grease a 9-by-5-inch (23-by-13-centimeter) loaf pan with butter. Place the twisted dough into the prepared pan. Cover the pan with plastic wrap and let the dough rise in a warm, draft-free place until the dough has risen at least 1 inch (3 centimeters) above the sides, about 1½ hours.

Make the crumb topping: In a small microwave-safe bowl, heat semisweet chocolate and butter on low power until the chocolate is shiny and the butter is melted; stir until smooth. Stir in flour and granulated sugar until crumbs form. Set aside.

Preheat the oven to 325°F (165°C) with a rack set in the middle position. In a small bowl, beat egg with 1 teaspoon (5 milliliters) water. Brush the top of the bread and sprinkle with the crumb topping mixture. Bake until an instant read thermometer inserted into the center of the bread reaches 190°F (88°C), 30–35 minutes. Let the bread cool on a wire rack for 10 minutes and remove from pan. Let the bread cool completely on the wire rack.