Coat two loaf pans with baking spray or butter (approx. 9" x 4" each), then line the bottom of each pan with a rectangle of parchment paper or aluminum foil.
Set up a large area for rolling out the dough and generously flour the surface - the dough can get sticky.
Remove half of the dough from the fridge but leave the other half in there.
Start rolling out the dough, halfway through rolling flip the dough over to make sure both sides get floured and the dough doesn't stick. You want to aim for a length of about 13"-15" (this is the side going away from you) and a width of about 10"-12", rolling it pretty thin.
Once the dough is all rolled out, spread half of the chocolate mixture evenly over the dough, make sure to leave a small border all around, about 1/2" - this will keep the chocolate from oozing out when begin rolling.
Brush the border farthest away from you with a little water.
Now it's time to roll! Begin at the side closest to you, rolling the dough in the direction away from you in a tight cigar. Seal the dampened end onto the log.
Lightly flour a baking tray and transfer the log of dough to the tray and freezer for about 5 minutes, this will help make the next step in the assembly process MUCH easier.
Repeat each of the above steps with the second dough.
When you place the second log in the freezer to chill, remove the first log.
Trim about 1/2" off each end of the log and then gently cut the log in half lengthwise, starting about 1/2" from the end, this keeps the log together at one end so you don't have to pinch the top ends together later. Once cut, adjust the two sides so that the cut sides are facing up. Lift one side over the other, forming a twist and trying to keep the cut sides facing up the entire time. This step can get messy, especially if you don't chill the dough long enough, but don't worry, just transfer the twist as best as you can into the prepared loaf pan. Take those pesky little ends that you cut off earlier and nestle them into openings in the pan, the dough will completely fill the pan once it is baked so don't worry about these little guys looking funky, you won't even notice them.
Cover the prepped babka with a damp tea towel and leave to rise for 1 - 2 hours at room temperature.
Repeat the process with the second loaf.