This chocolate babka is a stunning twisted loaf with rich swirls of cocoa goodness. Soft, tender, and topped with a sweet glaze, it makes a bakery-worthy treat at home.
In a large bowl or stand mixer, combine the flour, sugar, instant yeast, and salt. Add the warm milk, eggs, and vanilla extract. Mix with a dough hook on low speed until combined, then increase to medium and knead for 5 minutes.Add the softened butter a tablespoon at a time while the mixer runs, and continue kneading for another 5 to 7 minutes until the dough is smooth, elastic, and pulls away from the sides of the bowl.
Place the dough in a lightly greased bowl, cover tightly with plastic wrap, and refrigerate overnight, or for at least 8 hours. Cold dough is much easier to roll and fill.
Make the chocolate filling by melting the semi-sweet chocolate and butter together in a microwave in 30-second intervals, stirring between each. Once smooth, whisk in the cocoa powder and powdered sugar until completely combined. Let cool to a spreadable consistency.
Roll the chilled dough out on a lightly floured surface into a large rectangle, roughly 14x18 inches. Spread the chocolate filling evenly across the entire surface, leaving a 1/2 inch border along one long edge.Starting from the long side opposite the border, roll the dough up tightly into a log. Pinch the seam to seal.
Using a sharp bench scraper, cut the log in half lengthwise so the chocolate layers are exposed. Lay the two halves side by side with the cut sides facing up. Pinch them together at one end, then twist the two strands around each other, keeping the cut sides facing up as much as possible. Pinch the other end to seal.
Transfer the twisted babka into a greased 9x5 inch loaf pan. Cover loosely with a damp towel and let rise in a warm spot for 1 to 1.5 hours until puffy and nearly doubled.
Preheat your oven to 350°F (175°C). Bake the babka for 35 to 40 minutes until deep golden brown on top and the internal temperature reads about 190°F (88°C). If the top browns too quickly, tent with foil after 25 minutes.
While the babka bakes, make the simple syrup by heating the sugar and water in a small saucepan, stirring until the sugar dissolves. As soon as the babka comes out of the oven, brush the top generously with the warm simple syrup. Let cool in the pan for 15 minutes, then turn out onto a wire rack.