Ingredients

• 1/2 cup whole milk
• 1 1/4 ounces. envelope active dry yeast
• 4 large egg yolks
• 1 teaspoon vanilla extract
• 1/2 cup (1 stick) unsalted butter, melted, cooled, plus more
• 1/3 cup granulated sugar
• 1 teaspoon kosher salt
• 3 cups all-purpose flour, plus more
• 6 ounces bittersweet chocolate, finely chopped
• 1/3 cup (packed) light brown sugar
• 1 1/2 teaspoons ground cinnamon
• All-purpose flour (for surface)
• 1/4 cup (1/2 stick) unsalted butter, room temperature
• 1 large egg yolk
• Granulated sugar (for sprinkling)

Instructions

1. Heat milk in a small saucepan until warm. Transfer to a large bowl and whisk in yeast; let sit until foamy, 5-10 minutes.
2. Whisk in egg yolks, vanilla, and 1/2 cup butter. Add sugar, salt, and 3 cups flour; mix until a shaggy dough forms. Knead dough on a lightly floured surface until supple, smooth, and no longer shiny, 5-10 minutes. Transfer to a large buttered bowl. Cover and let sit in a warm place until doubled in size, 1 1/2-2 1/2 hours.
3. Filling and assembly: Mix chocolate, brown sugar, and cinnamon in a small bowl. Turn out dough onto a lightly floured surface; divide into three portions. Shape each into a 12"-long rope. Roll out each rope to a 12x6" rectangle about 1/8" thick. Brush with butter and top with chocolate mixture, pressing gently. Roll up to form a log; pinch seam to seal. Place logs, seam side down, side by side on a parchment-lined baking sheet. Pinch logs together at one end; braid, then pinch ends together and tuck under. Cover loosely and let sit in a warm place until 1 1/2 times larger, 1-2 hours. Preheat oven to 350°. Beat egg yolk with 1 tablespoon water in a small bowl. Brush dough with egg wash; sprinkle with granulated sugar. Bake until top is golden brown and "Babkallah" sounds hollow when bottom is tapped, 35-45 minutes. Let cool on a wire rack.
4. Mix chocolate, brown sugar, and cinnamon in a small bowl. Turn out dough onto a lightly floured surface; divide into three portions. Shape each into a 12"-long rope. Roll out each rope to a 12x6" rectangle about 1/8" thick. Brush with butter and top with chocolate mixture, pressing gently. Roll up to form a log; pinch seam to seal.
5. Place logs, seam side down, side by side on a parchment-lined baking sheet. Pinch logs together at one end; braid, then pinch ends together and tuck under. Cover loosely and let sit in a warm place until 1 1/2 times larger, 1-2 hours.
6. Preheat oven to 350°. Beat egg yolk with 1 tablespoon water in a small bowl. Brush dough with egg wash; sprinkle with granulated sugar. Bake until top is golden brown and "Babkallah" sounds hollow when bottom is tapped, 35-45 minutes. Let cool on a wire rack.