In a large bowl, dissolve yeast in warm milk with a pinch of the granulated sugar. Let stand 5 minutes, until frothy. Stir in remaining granulated sugar, eggs, 6 tablespoons softened butter, and salt.
Mix in cocoa powder and 3 cups of the flour, mixing and adding more flour as needed until mixture forms a soft dough. It should be sticky but not overly wet.
Turn dough out onto a lightly floured surface and knead until dough is smooth, about 6 minutes. The dough should feel soft and tacky. Shape into a ball and place in a large greased bowl. Cover and let rise for 1 hour, or until doubled.
Beat together 4 tablespoons softened butter, brown sugar, and cinnamon, creating a paste.
When dough has risen, turn onto floured surface, gently deflate and roll out to a large rectangle about 11x16 inches. Spread filling evenly over dough. Top sugar mixture with mini chocolate chips, pressing down gently to stick.
Roll up, starting with a long side, and pinch seams together to seal. Cut into 12 slices. (Keep a towel handy to clean knife after cuts, if needed.) You can also cut the dough into strips before rolling, then roll up each strip indvidually. Place rolls in a greased 9x13-inch baking pan. Cover and let rise until puffy, about 30 minutes. While the rolls are rising, preheat oven to 350°F.
Bake at 350°F for 30 minutes. Remove from oven and let cool slightly.
As the rolls cool, make the icing. Beat together powdered sugar and butter, then mix in milk and vanilla until smooth. Add more milk or sugar to get the consistency you prefer. Pour over the tops of the cinnamon rolls and serve.
Notes
You can mix these up the night before and pop them into the fridge after rising and shaping. When ready to bake, remove the rolls from the fridge, and let come to room temperature for at least 30 minutes. Bake at 350°F for 30 minutes.