To prepare the dough, in the large mixing bowl, sprinkle yeast over the warm milk. Let sit for 10 minutes.
In a small bowl, whisk sugar and egg until combined. Then slowly whisk in the yeast mixture.
In a mixer fitted with a paddle attachment, mix flour and salt. Run mixer on low and add egg mixture, mix until combined. Add butter and mix until incorporated.
Switch to a dough hook, let it knead the dough for 10 minutes on low speed. The dough will become sticky and stringy.
Cover the bowl with a tea towel and let sit in a warm place for an hour to rise.
To prepare the filling, pulse chocolate, salt and sugar in a food processor until the chocolate is very finely chopped with some parts almost powdery. Add butter and pulse machine until it’s distributed throughout the chocolate. Set mixture aside.
When the dough is risen, move it to a lightly floured surface and gently deflate it. Roll dough into a big rectangle, about 12 inches on the short end and 20 inches on the long end.
Evenly sprinkle the chocolate mixture over the dough.
Roll the short side of the rectangle tightly toward the other short end, forming a 12-inch log. Pinch the seam together at the end.
Slice it into 1-inch slices. Place the sliced rolls in a well greased muffin tins. Cover and allow to rise in a warm place for 30 minutes.
Preheat oven to 350F, and bake the rolls for 15 to 20 minutes, until golden brown. Serve warm or in room temperature.