In the bowl of a stand mixer fitted with the paddle attachment, combine the warm milk, sugar, and yeast and stir to blend.

Add the egg, flour, and salt, and mix on low speed until a sticky dough begins to form. Mix in the butter until incorporated and no chunks remain. Switch to the dough hook, and knead on low speed for 10 minutes. (The dough will be loose and sticky, avoid the urge to add more flour. It will all come together as it rises.)

Butter a large bowl, and place the dough inside. Cover with plastic wrap or a tea towel and let it rise until doubled, about 1- 1 1/2 hours.

In the bowl of a food processor add the chocolate (roughly chopped if in bars), sugar, cinnamon, and salt, and pulse until chocolate is fairly finely chopped and some parts powdery. Add the butter, and pulse until the mixture comes together. Set aside.

In a small bowl, combine the sugar and flour. Add the butter, and using a pastry cutter or 2 forks, cut the butter into the flour mixture until there are coarse crumbs and some larger clumps. Set aside.

Very generously butter a 12 cavity muffin tin.

Turn out the dough onto a lightly floured work surface, and gently deflate the dough. Let the dough rest for 5 minutes.

Roll the dough into a large rectangle, about 12x20-inches. Flouring as needed to prevent sticking.

Sprinkle the filling evenly over the dough. Roll the dough into a tight log, starting from the short end and rolling to the other end. Pinch the seams. Using a serrated knife or tooth floss (the best!), cut the dough into about 1-inch segments. Place each segment into the prepared muffin tin.

(If you want to bake the next day, cover with plastic wrap and refrigerate the dough overnight. Bake as directed without letting the dough rise again. Otherwise the dough can also be covered and frozen at this point).

Cover with plastic wrap and let rise for about 30 minutes.

Preheat oven to 350 degrees F.

Whisk the egg and cream together, and brush the tops of the buns with the egg wash. Sprinkle each bun with the streusel (if using) and bake for about 15 to 20 minutes, until puffed and brown around the edges.

Let cool on a wire rack in the muffin tin. These are easier to remove from the muffin tin if completely cool. But that being said, they are ridiculously good warm.