Ideally you should start the day before you want to bake the babka. Place the flour, sugar, yeast and salt in a large bowl or the bowl of a stand mixer fitted with a dough hook. Mix to combine.
Add the lukewarm milk and mix to form a rough dough. Knead until it forms a smooth ball then add the softened butter.
Continue to knead until the butter is fully incorporated and the dough is smooth and elastic. It may seem greasy and messy at first but keep kneading and I promise you the butter will all mix in and the dough will become smooth.
Once the butter is fully incorporated the dough should look smooth and silky and it should pull away from the sides of the bowl cleanly. It should still be soft and slightly sticky but if it seems too wet then you can knead in a little more flour; be careful not to add too much however.
Place the dough in a lightly oiled bowl, cover and place in the fridge to rise overnight.
The following day the dough should have doubled in size. Remove it from the fridge and set aside to warm up a little while you prepare the filling.
Place the butter, chopped dark chocolate, sugar, cocoa powder, cinnamon and a pinch of salt in a small pan over a low heat. Stir constantly until melted and smooth then remove from the heat and set aside to cool for 20 minutes. It should thicken slightly but still be spreadable. If it gets too firm then you can rewarm it very gently until it reaches a spreadable consistency.
Punch down the dough and roll it out on a lightly floured surface to a rectangle about 30x40cm / 11x15in.
Spread the filling evenly over the dough, leaving a 1cm border. Sprinkle over the chocolate chips, chopped chocolate or pecans and press them down gently.
Roll the dough up tightly from one of the long edges into a sausage. Use a sharp knife to cut the sausage in half lengthwise, exposing the filling.
Twist the two halves together into a braid. Grease an approx 11.5 x 21.5 cm / 4.5 x 8.5 in 2lb loaf tin and line it with baking parchment. Place the babka in the loaf tin, squishing it to fit if need be.
Loosely cover the tin and set aside to rise until doubled in size, about 1 ½ - 2 hours depending on how cold it is.
When the dough has nearly finished rising, preheat the oven to 180°C/350°F/gas mark 4.
Bake the babka for about 35-40 minutes until deep golden. A probe thermometer inserted into the centre should reach about 90°C/195°F.
While the babka is baking prepare the syrup. Place the sugar and water in a small pan over a medium heat. Stir until the sugar has dissolved and the syrup comes up to a simmer. Remove from the heat and set aside.
When the babka is ready, remove it from the oven and brush generously all over the top with the syrup. Keep going until you have used all of the syrup.
Leave the babka to cool in the tin for 15-20 minutes while it absorbs the syrup then turn it out onto a wire rack and leave to cool completely before slicing.