Make the dough: Heat the milk or water to about 110°F. (It should be lukewarm; if the liquid gets too hot allow it to cool slightly before moving on to the next step as hot liquid can kill the yeast).

Add 1 tablespoon of the sugar and stir to dissolve slightly. Sprinkle the yeast over and allow to stand for 10 minutes (it should start to bubble and fizz slightly).

Mix together the flour, remaining 2 tablespoons sugar and salt in a large mixing bowl or the bowl of a stand mixer. Make a well in the center of the flour and add the eggs, yolks, vanilla extract and yeast mixture. Knead (with your hands or a dough hook attachment) for about 5 minutes. 

Add the butter and knead for another 5 minutes, until the dough is elastic.

Cover the bowl with a dish towel and let rise for 45 minutes, or until doubled in size.

Meanwhile, prepare the filling. Put the butter, chocolate, cocoa powder, and sugar in a pot over medium-low heat. Stir until the mixture is full melted.

Roll out the dough on a clean, lightly floured surface (or on parchment paper) to a large rectangle. 

Spread the filling evenly over the dough. 

Cut the dough in half lengthwise, then slice it in 10-12 even pieces the other direction so there are about 24 rectangles. 

Cut each of these rectangles down the middle to create a triangle (there should be about 48 in total).

Roll up each triangle starting from the base of the triangle to form rugelach.

Transfer to a parchment-lined baking sheet. 

Allow the rugelach to rise once again for 30 minutes, until doubled in volume.

Preheat the oven to 350°F.

Meanwhile, make the sugar syrup by heating the water and sugar together over medium heat until the sugar is fully dissolved. Allow to boil for 1 to 2 minutes to thicken is just slightly. Remove from the heat and stir in the cinnamon.

Brush the sugar syrup over the rugelach and bake for 10 to 15 minutes, or until lightly browned (take out sooner rather than later as they continue to cook for a few minutes out of the oven, and you want a slightly doughy texture).

Remove from the oven and serve.