In a stand mixer, combine the warm milk, yeast, egg, sugar, salt, and 3 tablespoons of melted butter in the bowl. Switch to a dough hook and add in the bread flour and mix on "stir" speed for 3 minutes. You will need to scrape down the sides of the bowl once or twice, while mixing. Check the dough after 3 minutes.
Once the dough has doubled in size, punch it down in the center, then transfer it to a lightly floured surface. Use your hands to shape the dough into a rectangular shape, then roll it out into a 16×8-inch rectangle measuring. Spread the chocolate and cinnamon filling over the surface of the dough, leaving a border of about ¼-inch around the edge. Sprinkle the chopped chocolate over the top of the cinnamon mixture and use your hands to gently press the chocolate into the dough so that it doesn’t escape when you roll it up.
Grease a 9×13-inch baking dish, then arrange the Chocolate Cinnamon Rolls so that there is a little room in between each one. Cover with cling wrap and leave somewhere warm for 30 minutes. Meanwhile, preheat the oven to 350ºF. Once risen, brush the rolls with 2 tablespoons of melted butter.
They can be stored at room temperature for up to 3 days in an airtight container. They can also be stored in the freezer for up to one month. To reheat the rolls to enjoy, just pop them in the microwave or oven.