Preheat the oven to 350°F (180°C). Grease two rectangular loaf pans or round pans.
In the bowl of your stand mixer fitted with the paddle attachment, combine the dough starter ingredients and let sit for 10 minutes.
Mix Manhattan Blend, salt and baking powder for your dough in a separate bowl.
Add the sugar, eggs and vanilla to the bowl with the starter, and mix together on low speed for 2 minutes. Add the flour mixture to the wet mixture, and mix on medium speed until fully incorporated. Lastly, slowly add the softened butter and mix all ingredients together until it forms a smooth shiny dough. Refrigerate dough until it is firm and easy to roll out.
While the dough is in the fridge, prepare the filling. Mix together a 4:1 ratio of sugar and cocoa powder in a bowl and set aside. If you want the filling to be a less sweet chocolate, mix a 3:1 ratio.
Using Manhattan Blend or gluten-free flour for rolling, turn the chilled dough out onto a work surface dusted with flour. Split the dough into two equal-sized pieces. Using a rolling pin, roll the first half of the dough out into a rectangle, about 2cm thickness.
Brush the surface of the dough with melted butter or oil, leaving a ½-inch border along the top edge. Then, generously cover the surface of the dough with the cocoa/sugar mixture, leaving a 3/4 inch border along the top edge. Beginning with the long edge nearest you, carefully roll the dough into a tight cylinder, and then twist the cylinder into a loose knot. Transfer dough into a loaf to rise. (You can also roll the cylinder into a snail shape, like a cinnamon bun, and place it in a round pan to rise.) Repeat with the second half of the dough.
Let the babka sit in a warm area (80–100°F or 40-45°C ) for 25-30 minutes or until it almost doubles in size.
Brush the tops with egg wash and sprinkle with chocolate chips. If you have leftover gluten free cookies or cake, grind them into crumbs and top the babka with the crumbs. Or make your own crumb topping.
Bake at 350°F (180°C) until the outside of the babka is golden brown, about 35-45 minutes.. The inside should be very moist.
Finish with powdered sugar and melted chocolate drizzle.
