In a large bowl, mix together the warm milk, yeast, and sugar until dissolved. Let sit for 10 minutes until frothy. Then add the eggs and egg yolk, vanilla extract, flour, salt, and butter. Stir together until a dough forms.
Turn the dough out onto a floured surface and knead for 8-10 minutes until smooth and elastic. Place the dough in a greased bowl and cover with plastic wrap. Let it rise in a warm place for 1 hour or until doubled in size.
In a medium bowl, mix together the semisweet chocolate chips, sugar, and cinnamon. Set aside.
Once the dough has doubled in size, punch down and roll out on a lightly floured surface. Brush with melted butter and spread the filling evenly over the dough. Sprinkle with chopped pecans, if desired.
Roll up from one of the long sides, jelly-roll style, into a log shape. Cut in half lengthwise and twist each half together.
Place the twisted babka onto a parchment-lined baking sheet. Cover with plastic wrap and let rise in a warm place for 45 minutes or until doubled in size. Preheat your oven to 350°F and bake for 25-30 minutes or until golden brown.
While the babka is baking, make the syrup. In a small saucepan over medium heat, combine the chocolate, butter, sugar and water and bring to a boil until thickened. Once the babka is out of the oven, cover it with the syrup and let it cool before serving.
Cut it into pieces & serve.