Directions:

1. To the bowl of your stand mixer filled with the dough hook, add 3 cups of the flour, sugar, yeast, and salt. Mix on low speed to combine. In a small saucepan, over medium/low heat, melt the butter until just melted. Stir in the milk and let mixture heat through for 1-2 minutes. Remove from heat and let mixture sit for a few minutes to cool off a bit. You want this mixture to be warm, not hot (or else you can kill the yeast!)
2. With the mixer on low, pour the milk/butter mixture into the dry ingredients. Add the vanilla extract. Mix for 1 minute or until incorporated. Add the eggs, one at a time, and mix until the eggs are incorporated. Add the remaining 3/4 cup of flour and mix for 5-6 minutes on medium speed, until the dough is elastic and sticky. Place the dough in a large, greased bowl – you can use cooking spray, butter, or a neutral oil. (You may want to butter/oil your hands to make working with the sticky dough a little bit easier). Cover the bowl with plastic wrap, and place a kitchen towel over the bowl. Place in a warm spot and allow dough to rest until doubled in size, about 3 hours.
3. Liberally grease two 9×5 loaf pans with butter and line with parchment paper, leaving some overhang on the side for easy removal. Set aside.
4. To make the filling, melt the butter and chocolate in a large microwave safe bowl in 30-second increments, mixing after each 30 seconds, until melted, about 1½ minutes total. Add the sugar, chocolate sweet tahini, cocoa powder, and cinnamon, and whisk until smooth. You may need to transfer to a larger bowl.
5. Once dough has risen, turn dough out onto a floured countertop, and divide dough in half. Making sure that your countertop is liberally floured, roll one dough half into a rectangle about 10 x 14 inches, with the longer side closest to you. Spread half of the filling over the rolled dough using an offset spatula, leaving about 1 inch on the edges. Starting with the side closest to you, roll the dough into a tight roll. Place the roll seam side down on a cookie sheet. Repeat with remaining dough and filling. 
6. Place rolls in the freezer for 15-20 minutes while you prepare the syrup. In a medium saucepan, combine the water and sugar. Bring to a boil. Once boiling, stir for 20-30 seconds, until sugar has melted. Set aside to cool completely.
7. Remove rolls from the freezer and place on a floured countertop. Using a sharp knife, cut the rolls in half, lengthwise. With the cut sides facing up, gently pinch together the ends on one side, then braid the two pieces together, crossing over each piece alternately, then pinching the other end together to secure. Place in one of the loaf pans, and repeat with the second roll. If the dough looks messy in the pan, don’t worry! It will be ok.
8. Cover pans loosely with plastic wrap or a kitchen towel and let rise for 1½ hours. When they have 30 minutes of rising time left, preheat the oven to 350F.
9. Bake the babkas for about 35-40 minutes, or until golden brown on top, and a toothpick or skewer inserted in the center comes out clean, with no dough attached. Brush the babkas with the cooled syrup. Let the breads cool completely in the pans, then remove, slice, and enjoy.