On the stove, warm the milk and add in 1 tbsp of sugar. Sprinkle the dry yeast atop the mixture and allow it to sit for 10 minutes until you can see bubbles forming.
Using a stand mixer, mix flour, 2 tbsp sugar and salt. Then, add in the eggs, egg yolks, vanilla extract and your yeast mixture into a small well in the center. Knead the formed dough together for 5 minutes.
Add in 3.5 tbsp of your softened butter and continue to knead. Within another 5 minutes or so, the dough will feel like elastic. Cover the bowl to rise until doubled in size, roughly 40 minutes.
As the dough proofs, begin to make the chocolate filling. Put 7 tbsp butter, chocolate, cinnamon and sugar into a saucepan on the stove.
Roll out the dough on a lightly floured surface into a rectangle. Spread your filling atop the dough and cut into long triangles. Roll each one up and transfer to a parchment-lined baking sheet. Let the rugelach rise until doubled in volume.
Preheat oven to 350 degrees while you make a sugar syrup on the stove with dissolved sugar and water. Brush the rugelach with the syrup and bake for 10-15 minutes.
