Whisk flour and yeast together in bowl of stand mixer. Add milk; eggs; orange zest, if using; and vanilla. Fit mixer with dough hook and mix on medium-low speed until cohesive dough comes together and no dry flour remains, about 2 minutes. Turn off mixer, cover bowl with dish towel or plastic wrap, and let dough stand for 15 minutes.

Add sugar and salt to dough and knead on medium speed until incorporated, about 30 seconds. Increase speed to medium-high and, with mixer running, add 6 tablespoons butter 1 piece at a time, allowing each piece to incorporate before adding next, about 3 minutes total, scraping down bowl and dough hook as needed. Continue to knead on medium-high speed until dough begins to pull away from sides of bowl, 7 to 10 minutes longer.

Transfer dough to greased large bowl. Cover tightly with plastic and let rise at room temperature until slightly puffy, about 1 hour. Refrigerate until firm, at least 2 hours or up to 24 hours.

Just before removing dough from refrigerator, combine chocolate and butter in medium bowl. Microwave at 50 percent power, stirring often, until chocolate is fully melted and smooth, about 2 minutes. Stir in sugar, cocoa, and salt until combined; set aside.

Adjust oven rack to middle position and heat oven to 325 degrees. Grease 8½ by 4½-inch loaf pan with 1 tablespoon butter. Remove dough from refrigerator and turn out onto lightly floured counter. Using floured rolling pin, roll dough into 18 by 12-inch rectangle, with short side parallel to edge of counter.

Using offset spatula, spread chocolate mixture evenly over dough, leaving ½-inch border along top edge. Beginning with edge nearest you, tightly roll dough away from you into even 12-inch log, pushing in ends to create even thickness. Pinch seam to seal.

Using greased serrated knife and slicing in only 1 direction, gently cut log in half lengthwise and lay halves next to each other cut sides up. Forming tight twist, cross left log over right log. Continue twisting, 5 times total, keeping cut sides facing up as much as possible. Pinch ends together and carefully transfer to prepared loaf pan cut sides up. Tap loaf pan on counter to pack dough into pan.

Set wire rack in rimmed baking sheet and center loaf pan on wire rack. Bake for 30 minutes. Remove from oven and cover babka with aluminum foil. Return to oven and continue to bake until center registers 200 to 205 degrees, 50 minutes to 1 hour.

Meanwhile, combine sugar and water in small saucepan and heat over medium heat until sugar dissolves. Set aside off heat.

Remove babka from oven. Leaving babka in loaf pan, brush syrup evenly over entire surface of hot babka (use all of it). Let cool in loaf pan on wire rack for 1 hour. Carefully remove babka from pan and let cool completely on wire rack, about 2 hours. Slice 1 inch thick and serve.