Chocolate Rye Babka

To prepare dough: Combine milk and yeast in the bowl of a stand mixer fitted with the dough hook attachment, stirring briefly to incorporate. Let stand for about 5 minutes. Add all-purpose flour, rye flour, egg, 1 tablespoon sugar and 3/4 teaspoon salt. Mix on low speed until the dough comes together off the sides of the bowl, about 2 minutes. Add softened butter and continue to mix until a soft, smooth, elastic dough forms and you can hear it slapping on the sides of the bowl, 8 to 10 minutes.

Cover the dough with a clean kitchen towel and let it rest at room temperature until puffy and roughly doubled in size, 1 to 2 hours. (Alternatively, you can refrigerate the dough for 12 hours until you're ready to shape and bake your babka. This adds flexibility and also makes it easier to roll out the dough and spread the filling onto it when the time comes.)

To prepare filling: About 15 minutes before you're ready to shape your babka, heat butter and 1/4 cup sugar in a medium saucepan over medium-low heat just until the butter has melted and the sugar has dissolved a bit, about 3 minutes. Remove the pan from the heat and add chocolate. Let the warmth of the butter and sugar melt the chocolate for a minute or so, then stir until smooth yet still gritty. Stir in cocoa, cinnamon, espresso powder (or vanilla) and salt. Set aside to cool to room temperature, about 10 minutes. 

To prepare babka: Lightly coat a 9-by-5-inch loaf pan with cooking spray.

Roll the dough on a lightly floured surface into a 12-by-15-inch rectangle. Spread the cooled filling evenly over the dough to the edges. Sprinkle with nuts and cacao nibs, if using. Starting from one long side, roll the dough up into a tight log. If it's stretched longer than 15 inches, squish it back with your hands. 

Using a sharp knife, cut the log in half lengthwise. With the cut sides facing up, twist the two halves together like a rope. Lift the twisted dough and scrunch it up so that you can fit it into the prepared pan. Cover and let the babka rise until it has just crested over the edge of the pan, 45 to 90 minutes.

Preheat oven to 350°F.

Bake the babka until the top is a deep golden brown and an instant-read thermometer inserted into the center of the loaf registers 190°F, about 40 minutes.

Meanwhile, prepare glaze: Combine sugar and water in a small saucepan. Bring to a simmer over medium heat. Cook, stirring occasionally,  until the sugar has dissolved, 1 to 2 minutes. Remove from the heat.

Brush or spoon the glaze on the top of the babka immediately when it comes out of the oven. Allow the babka to cool in the pan for about 15 minutes, then transfer it to a wire rack to cool completely, about 45 minutes, before cutting.