Place flour, sugar, yeast, and salt in a standing mixer fitted with the dough hook and mix on low speed until combined. Add eggs and water, and mix on medium speed until dough comes together, 2-3 minutes. Add butter, adding a few cubes at a time, mixing on low-medium speed, until incorporated. Continue mixing for about 10 minutes on medium speed, until dough is completely smooth, elastic and shiny. It would look soft and might be sticky – that’s ok. During mixing, you will need to scrape down the sides of the bowl.

With floured hands, transfer dough to a large mixing bowl brushed with oil, cover with plastic wrap, and leave in the fridge for at least half a day or overnight. The dough may not look like it has risen much, that's ok. If placed a long time in the fridge the dough can become hard, that’s ok, too; leave it at room temperature until it’s easy to work with, 30-60 minutes.

Grease two loaf pans (I usually use 9×5 inch pans but a bit smaller or larger pans would work too) with oil and line the bottom and sides of each pan with parchment paper for easy release later on. Set aside.

In a medium saucepan place chocolate, butter, heavy cream, cocoa powder, sugar, and salt, and bring just to a boil over moderate heat. Reduce heat to low and mix until melted and completely smooth. Transfer to a bowl and set aside to cool. It would thicken and become spreadable as it cools. You can place it in the fridge for a short while until you get a spreadable consistency.

Divide dough in half. Roll out dough with a rolling pin on a lightly floured surface and shape into a rectangle measuring 16x12 inches (40x30 cm). Position dough so that a long side is closest to you. Using an offset spatula, spread half of the chocolate mixture over the rectangle.

Use both hands to roll up the rectangle like a roulade, starting from the long side closest to you and ending at the other long end. Press to seal the dampened end onto the roulade, then use both hands to even out the roll into a perfect thick cigar. Rest the cigar on its seam.

Using a serrated knife, gently cut the roll in half lengthwise, starting at the top and finishing at the seam, essentially dividing the log into two long even halves, with the layers of dough and filling visible along the length of both halves. With the cut sides facing up, gently press together one end of each half, then lift the right half over the left half. Repeat this process, but this time lifting the left half over the right, to create a simple two-pronged plait. Gently squeeze together the other ends so that you are left with the two halves, intertwined, showing the filling on top. Carefully lift the cake into a loaf pan. Don’t worry if there are gaps in the pan since the cake will rise and will eventually look fine, even if it looks messy at this point. Cover the pan loosely with plastic wrap and leave to rise at room temperature for 1 to 1½ hours until almost doubled in size. Repeat to make the second cake.

Preheat oven to 350°F/175°C, making sure to allow plenty of time for it to heat fully before the cakes have finished rising. Remove plastic wrap, and place cakes on middle rack of oven. Bake 30-35 minutes, until golden brown on top. If you have a thermometer, you are looking for an internal temperature of about 200ºF/93ºC degrees.

While the cakes are in the oven, make the syrup. In a small saucepan over medium heat, bring water and sugar to a boil. As soon as the sugar dissolves, remove from heat and set aside to cool. As soon as the cakes come out of the oven, brush the syrup over them. Use all of the syrup, even if it looks a lot. Let cakes cool until they are warm, then remove from pans and let cool completely before serving. Serve warm or at room temperature.

Babka will stay fresh for 24 hours in an airtight container at room temperature. Do not place it in the fridge. It also freezes well for up to 2 months. To thaw, leave at room temperature for 2 hours, or overnight in the fridge.