Continue mixing on medium speed for about 10 minutes - at first, you'll think there's too much butter for the dough to ever solidify, but after a while you should find that the dough becomes quite smooth and looks like a regular bread dough.

Lightly oil a bowl or proofing tub - whichever will fit in your fridge - and set the dough in it. Lightly cover the dough with plastic (this will prevent the top of the dough drying out) and leave in the fridge overnight. It will grow a little in size as it proves.

Make the filling: Put the butter and chocolate into a microwavable bowl and heat for 10-second increments, mixing after each burst, until fully melted and smooth. Stir in the sugar, cocoa and cinnamon.

Split the dough into two equal halves. Put one half back into the fridge in cling wrap while you prepare the first loaf.

Lline two 9"x4" loaf pans with parchment paper. Take half of dough from fridge (leave the other half chilled). Roll the dough out on a well-floured counter to roughly a 10"x12" rectangle. Don't worry if it's not exact.

Spread half of the melted chocolate evenly over the dough, leaving an uncovered border of half an inch. Brush one short side with water (this will help it stick once it's rolled). Starting at the opposite side, roll the dough into a tight spiral and crimp the wetted side onto the roll to seal it. Wrap in cling wrap and place in the freezer for 15 minutes to chill while you make the second loaf.

The final step is to assemble the loaf. Trim the ends of the chilled loaf, about a half inch, to give you a clean edge and reveal the spiral inside. Cut the roll in half along its full length, so that the insides are exposed, and then lay the two cut halves side by side on the counter with the chocolate side on top. Pinch the top ends gently into each other, then lift one half over the other, keeping the chocolate side on top, and again push the bottom ends into each other to maintain the structure. Lift the whole loaf carefully into your loaf pan. Repeat with the second loaf.

Cover the pans with a damp tea towel and leave to rise another 1 1/2 hours at room temperature.

Finally, bake in a preheated oven at 375°F (190°C) for about 30 minutes (check with a skewer after 25 minutes; you don't want to see dough on it). You can put it back in for 5 minutes at a time if it's not quite ready.

Meanwhile, add the sugar and water to a small pan and simmer for a couple of minutes. Once the babkas are out of the oven, brush the syrup onto the top of both. Leave to cool on a rack for 20 minutes.