For the brioche:

1. Heat the milk to 110 degrees F and stir in yeast and sugar. If using active dry yeast, allow to sit for 15 minutes. If using instant yeast, simply move on to the next step.
2. Mix the eggs and vanilla into the milk yeast mixture.
3. Combine flour and salt together in the bowl of a stand mixer fitted with a dough hook.
4. Pour the wet ingredients into the dry ingredients and mix on low-medium speed for about 3 minutes, or until it forms a ball around the hook.
5. Add in one piece of butter at a time, allowing it to fully incorporate before adding the next. Once all of the butter is incorporated, turn the mixer up to medium-high speed and mix for 8-10 minutes. To check if the dough is ready, tear off a small piece and carefully spread it out to see if you can see the light through it without it tearing. If it tears, mix for another minute and check again.
6. Once the dough is ready, transfer it to a lightly greased bowl, cover, and place in the fridge to proof overnight. Alternatively, you can proof the dough in a warm environment for one hour or until doubled in size, then assemble and bake the same day.

For the chocolate tahini filling:

1. Just before you are ready to assemble the babka, make the filling.
2. In a small saucepan, melt the butter over low-medium heat.
3. Add the sugar and stir until it begins to bubble along the sides and the sugar has dissolved. 
4. Remove the saucepan from heat, then stir in the chocolate until melted and fully combined. 
5. Lastly, stir in the tahini and cocoa powder.
6. Set aside while you roll out the dough.

For assembly:

1. On a lightly floured surface, roll the dough out into a rectangle about 10" x 15".
2. Spread the chocolate tahini filling evenly all over the dough, leaving a small border around the edge. 
3. Roll the dough up from the short side. To make it easier to cut and shape, place the rolled up dough in the freezer for about 20 minutes. 
4. Cut the ends of the dough off - you can use your loaf pan to measure how much to cut off. Then, cut straight down the middle, leaving you with two exposed pieces. Braid the dough, placing one piece over the other, keeping the layers facing upwards.
5. Transfer the babka to a loaf pan lined with parchment paper. You may need to squish it up slightly so it will fit. Cover with plastic wrap and allow to rise in a warm environment for about one hour.
6. Towards the end of the proof time, preheat the oven to 350 degrees F (180 degrees C).
7. Bake for about 35-40 minutes or until the inside of the babka registers 190 degrees F (90 degrees C).
8. While still warm, brush the sugar syrup all over the top of the babka, then allow to cool completely before cutting.

For the sugar syrup:

1. Combine all ingredients in a small saucepan and boil for 4-5 minutes.
2. Allow to cool before brushing on the warm babka.