• The evening before: Mix the starter with flour and water. Knead it together into a soft dough ball. Place it in a lightly greased jar and cover with a lid. Let it rise for 8-10 hours until it doubles and is domed on top.
• The following day: Add the flour, sugar, salt, eggs, milk, and starter in a stand mixer bowl with a dough hook attachment. Turn the mixer on low speed and combine until it forms a thick but slightly sticky dough.
• Add in the cubed butter, a few pieces at a time.
• Turn the mixer on medium sped and mix it for 10-15 minutes. Mix until the sticky dough strengthens and comes together, and pull away cleanly from the sides of the bowl.
• Let the dough rest for a few minutes, then see if you can stretch a little bit out thinly so it’s see-through, without tearing. This is called the ‘windowpane test’ and shows proper gluten development.
• Place the dough into a greased bowl and cover it. Place it in a warm spot, ideally around 25°C/ 76°F, but under 30°C / 86°F so the butter doesn’t melt. Let the dough rise until it has bulked out by at least 50-60%. This will take around 5-7 hours, depending on temperature. This dough will struggle to rise if the temperature is too cool.
• Don’t rush this rise. It’s an important one and will set the standard for any future rising the dough will do. Once risen, ensure the dough is covered tightly and place in the fridge until you’re ready to shape it.

Shaping

• Grease and line two 9×5 inch loaf pans with parchment paper
• Melt chocolate and butter in a saucepan over low heat. Once melted, remove from the heat, and stir in powdered sugar and vanilla while it’s still warm.
• Pour into a bowl and let it cool until it becomes a spreadable paste. You can place it in the fridge to speed this up, but check on it and stir it regularly so it doesn’t stiffen too much. If it does, warm it up slightly to thin it out again.
• Pull the cold dough from the bowl (it will be stiff) and weigh it on a kitchen scale. Cut it into two equal pieces.
• Take one piece and use a rolling pin to roll it into a rectangle measuring around 11×14 inches (27×35 cm).
• Dollop half the chocolate mixture on. Use an offset spatula to spread it out, leaving a ½ inch (1cm) border around all the sides.
• Roll it up, from the long side upwards, into a log. Place it in the fridge for 15 minutes. Repeat with the other dough piece and the remaining filling.
• Shaping option 1- Once chilled, slice a log in half lengthwise with a sharp knife to expose the chocolate filling.
• Take the top two ends and fold them over. Lift the left half over the right half, then the right over the left, and repeat all the way down, exposing the chocolate filling. When you reach the end, tuck the dough underneath to compact the loaf. Place the braid into a prepared loaf pan.
• Shaping option 2- Twist the log up its length, then take the two ends and twist them over each other. See the post for pictures.
• Loosely cover the babkas and let them rise in the loaf pans in a warm spot until they have doubled in size - around 4-6 hours depending on temperature.

Baking

• Preheat the oven to 350°F/180°C and place a rack in the middle. Bake the babkas for approximately 35-40 minutes until deep golden brown. If they are browning too much, loosely cover the tops with foil.
• While the babkas are baking, prepare the sugar syrup to glaze them. Combine the sugar and water in a small saucepan over medium heat. Bring it to a simmer until the sugar dissolves, then remove it from the heat.
• Once the babkas are removed from the oven, brush them liberally with the syrup.
• Let them cool in their pans for 20 minutes, then remove them and let them cool further on a wire rack. Serve the babka at room temperature or slightly warm.