Preheat the oven to 350°F. Make standard challah dough and take a third of that batch for this babkallah. Using a double boiler (or a heat-safe bowl over a pot of water), heat the chocolate and butter until melted. Remove from the heat.
Add the powdered sugar, cocoa powder, and salt into the melted chocolate and stir until combined.
Roll your challah dough out into a large rectangle. Pour the chocolate mixture in a thick line over the center of the rolled out dough. Using a spatula, spread it evenly over the entire rectangle, leaving about a 1" margin.
Roll the challah dough and stop about 2" before you're finished.
Instead of using the rolled part of the dough to roll the rest of the way, pinch the clean, plain challah dough on the end and lift it up and over the roll, lightly pinching it with the dough that it meets on the roll to secure it.
Place the rolled up dough onto a cookie sheet and put into the freezer for 15 minutes (this will make it easier to cut). Using a bench scraper or a knife, cut in the center of the dough along it's entire length, halving the roll. Quickly turn the halves so that they are laying cut-side up.
Twist the two strands, keeping the cut-sides facing up the entire time.
Gently lift your babkallah and settle it down into a greased loaf pan. Bake for 45 minutes. If it browns too quickly, cover with foil.
Remove from the oven and make sure that the babkallah is cooked through. You'll know it is when you gently try to tease braids apart (on the end or somewhere else inconspicuous), there is no evidence of doughiness, and the braids want to stay intact, almost like there are strands of gluten holding them together.
Mix the remaining sugar with the boiling water and mix until dissolved. Brush over the entire babka while it's still hot. Let the babkallah cool for about 10-15 minutes, or until just warm. Garnish the top with the Maldon salt. Remove from the loaf pan and put on a serving plate. Serve warm.