Step 1

Make the cake: Sprinkle yeast and a pinch of granulated sugar over milk in a medium bowl. Let stand until foamy, about 5 minutes.

Step 2

Whisk together remaining 1/4 cup plus 2 tablespoons granulated sugar, the egg, and yolk. Whisk into yeast mixture.

Step 3

Combine flour and 1/2 teaspoon salt in the bowl of a mixer. Add egg mixture. Beat on low speed until almost fully combined, about 30 seconds. Switch to the dough-hook attachment. Add butter. Beat until smooth, soft, and slightly sticky, about 10 minutes.

Step 4

Butter a large bowl. Turn out dough onto a floured surface; knead a few times until smooth. Place in bowl, turn to coat, and cover with plastic wrap. Let stand in a warm place until doubled in volume, 1 to 1 1/2 hours.

Step 5

Make the filling: Combine chocolate, granulated sugar, and cinnamon. Cut in butter with a pastry cutter or rub in with your fingers until combined.

Step 6

Punch down dough. Transfer to a floured work surface. Let stand for 5 minutes. Roll out to an 18-inch square (about 1/8 inch thick). Brush edges with egg wash. Spread filling over dough, reserving 1/2 cup and leaving a 1-inch border. Tightly roll dough like a jelly roll. Pinch seam to seal, and fold in half to bring ends together to form a U. Twist 2 or 3 times to "braid." Butter a 5-by-10-inch loaf pan, and line with parchment, leaving 1-inch overhangs; butter parchment. Transfer dough to pan. Brush top with egg wash.

Step 7

Make the crumb topping: Combine confectioners' sugar, flour, and butter. Sprinkle topping and reserved 1/2 cup filling over cake.

Step 8

Preheat oven to 350 degrees. Drape plastic wrap over dough. Let stand in a warm place until risen by half, 20 to 30 minutes.

Step 9

Remove plastic wrap, and bake, rotating halfway through, until golden, about 55 minutes. Reduce oven temperature to 325 degrees. Bake until deeply golden, 15 to 20 minutes more (cover with foil if top gets too dark). Transfer pan to a wire rack. Let cake cool.