1. Place the yeast, warmed milk, sugar and 2/3 cups of flour in the bowl of a stand mixer. Mix them just until combined. Then put the bowl aside to rest in a warm place for 10-15 minutes.
   
2. Once the yeast mixture has bubbles and begun to expand, put it on the mixer with a dough hook.
   
3. On low speed add the butter into the yeast mixture, then the eggs and salt. Add 4 cups of the flour gradually, until the mixture is well mixed, then raise the speed to medium-high and knead the dough until it is smooth. If the dough does not come off the sides, add additional flour by tablespoons until it easily turns into a ball at the center of the mixer. (I used about 2-3 additional tablespoons of flour.)
   
4. Cover the dough with a clean towel and refrigerate the dough overnight. If you're in a rush, the rise can be lessened to 6 hours.
   

Chocolate Tahini Filling

1. Mix the melted butter, confectioners sugar, bittersweet chocolate, tahini, and cocoa into a smooth mixture.
2. Set aside the chocolate mixture and separately the nuts and 2 tablespoons of sugar. Those 3 elements constitute the filling.

Forming the Babka

1. Butter or oil two loaf pans. Line them with parchment paper with at least 2-3 inches overhanging on the long sides of the pans. Those sides will help you release the babkas once they are cool.
2. Take the dough out of the refrigerator, cut it in half. Put one half back in the refrigerator and roll the other half into an 11" x 15-18" rectangle on a lightly floured surface. The long side of the rectangle should be facing you, with the shorter sides perpendicular to you.
   
3. Using a knife or spatula, spread the chocolate tahini filling evenly over the rectangle of dough. Leave just a small border at the top (the long side farther from you). Sprinkle half of the nuts over the filling and then 1 tablespoon of the sugar.
   
4. Roll the dough, starting with the long side near you. Do it as tightly as you can without punching or otherwise damaging the roll.
   
5. Seal the roll by brushing a tiny bit of water on the small border that you left unfilled at the top. and press very gently. Turn the roll so it is perpendicular to you. Then slice it down the center with a sharp knife and open it so the two halves are side-by-side. The two halves may not stay together well - that's OK. Just keep them close together and don't move them much.
6. Braid the two halves as well as you can, lifting one half over the other. Try not to stretch them out lengthwise.
   
7. Gently place the braided babka into one of the parchment-lined loaf pans and repeat the process with the other half of dough and filling. If you have to squish the braid into the loaf pan, that's fine.
   
8. Cover the two loaves with a clean cloth, place them in a warm place and let the dough rise a second time for 1 & 1/2 -2 hours. Pre-heat the oven to 375 degrees.
   
9. Bake the babka for about 30 minutes, until a skewer comes out with no damp dough. (It's ok if chocolate attaches to the skewer, just no unbaked dough.)

Orange Simple Syrup

1. Either while the loaves or rising or while they bake, make the orange syrup. Mix the orange juice, honey, sugar and (optional) drops of orange blossom water. Heat the mixture just to a boil, watching to make sure it doesn't boil over. Skim off any foam, so that the mixture becomes a clear orange liquid. Add the grated orange rind and mix the rind throughout.
2. When you take the loaves out of the oven, immediately put them on a rimmed cookie sheet that is larger than the loaf pans. Spoon the syrup over the loaves, trying to get it into crevices.
   
3. Let the loaves completely cool before attempting to remove them from the loaf pans.