Finely chop the chocolate and add it to a heat proof bowl.
Heat the cream until boiling and pour it over the chopped chocolate. Leave it to sit for 5 minutes, then whisk together to melt all the chocolate.
Set it aside to firm up.
Mixing the Dough
Heat the milk until it is just lukewarm.
In a large stand mixing bowl, add flour and sugar, and whisk together. Make a well in the center and pour in the yeast and then the milk. Slowly stir a little flour into the milk and yeast mixture. Cover the bowl with a towel and set it somewhere warm to rest for 10 minutes.
After the 10 min have passed, give the bowl a quick stir with a rubber spatula.
Add the softened butter, egg, egg yolk, and vanilla extract. Mix together with the dough hook on low speed for 10 min or until the dough is a smooth cohesive ball of dough.
Shape the dough into a ball and place in a greased bowl. Cover the bowl with a towel / or proofing bag and set it somewhere warm to rest for 30 minutes.
After the 30 minutes have passed, add in the chocolate chips and knead by hand until the chips are fully dispersed in the dough.
Assembling & Baking
Preheat the oven to 350 F.
Divide the dough into 15 equal pieces by weight.
Shape each piece of dough into a ball and then flatten each ball into a 5" - 6" disk. Working with one piece of dough at a time, scoop a tablespoon sized scoop into the center of the disk and then pinch the dough up and around the chocolate ganache and seal up the ball.
Place each ball into the greased 9 x 13 pan. They should be arranged so that there are 3 balls across and 5 down the pan. Once all of the buns have been shaped, cover the pan and place somewhere warm for another 30 min.
After the 30 min have passed, bake the pan of buns on the center rack of the oven for 20-25 min until they reach a deep golden brown.
To create a shiny top, brush the buns with a teaspoon of melted butter.
Serve warm with the optional vanilla sauce.
Notes
These are best eaten fresh out of the oven, but they can be stored in the fridge in an airtight container and then reheated individually in the microwave!