Whisk the warm milk, 2 Tbsp sugar, and the yeast together in a stand mixer with the paddle attachment. Cover and let sit for 5 minutes, or until a foam forms on the top.
Add the remaining 2 Tbsp sugar, butter, eggs, salt, cocoa powder, and 1 cup of flour. Mix on medium speed for 1 minutes. Scrape the sides of the bowl and add the remaining flour and mix on low speed until a soft dough forms. The dough will pull away from the sides of the bowl.
Replace the paddle attachment with the dough hook. Knead the dough on medium-low speed for 6-8 minutes, or until the dough is smooth and elastic. Pause every few minutes to check the consistency. Add flour a few tablespoons at a time if the dough is still sticky and continue kneading.
1st Rise: add the dough to a lightly greased bowl. Cover and rise in a warm area for 2 hours, or until the dough is roughly double in size.
Lightly grease a 9"x13" baking dish. Roll out the dough to a roughly 22"x14" rectangle (this doesn't have to be super precise!). If you have trouble rolling out the dough, let it rest for 5-10 minutes to let the gluten relax and try again.
Filling: mix the brown sugar, cocoa powder, cinnamon, vanilla, and espresso powder in a small bowl. Spread the butter onto the rectangle of dough. Sprinkle the filling over the butter, followed by the chopped chocolate.
Form the Rolls: tightly roll the dough into a 22" long roll. Cut the dough in half and in half again, making 4 smaller rolls. Cut the 4 smaller rolls into thirds to make 12 large rolls. For 16 smaller rolls, cut the 4 smaller rolls in half and then half again.
2nd Rise: cover the rolls and let them rise in a warm area for 1 hour, or until roughly double in size.
Bake: Preheat the oven to 350℉. Bake the rolls for 22-26 minutes. Bake time may vary depending on how gooey or baked you like your cinnamon rolls. Cover the rolls loosely with foil about 15 minutes into baking to prevent them from browning too quickly o the top. Once done, cool the rolls in the pan on a cooling rack for 10 minutes.
Frost the rolls: while the rolls cool, make the chocolate frosting and/or espresso glaze. Frost the rolls and drizzle the glaze over the top of the rolls, if desired. Serve and enjoy!