Combine milk, eggs, butter, yeast, and granulated sugar in a large bowl and mix with a whisk or paddle attachment of stand mixer
Add flour and salt on top of the wet ingredients, and combine with hand or dough hook attachment of stand mixer until well combined. The dough will be wet and slack
Cover with floured kitchen towel or paper towel and rest for 20 - 30 minutes so flour can absorb liquid ingredients
Turn out dough onto floured surface and knead for 5-10 minutes until it has tightened and smoothed. Check to see if gluten has developed with the window pane test (see above)
Cover with floured kitchen towel or paper towel and let rest for 1.5 - 2 hours for the first proof. The dough should approximately double in size and should be light and springy to the touch
Combine cinnamon and brown sugar for filling while the dough proofs and grease your loaf pans
Turn out dough onto floured surface and divide into two. Stretch one portion of the dough evenly with your hands until it is approximately an 8 in. by 12 in. rectangle. Distribute half the cinnamon and brown sugar mixture evenly across the dough rectangle. Then, distribute half the chopped chocolate on evenly on top of the cinnamon and sugar. Roll the rectangle toward you on the shorter 8 in. side, so that you have a 12 in. coiled cylinder, making sure not to roll too tightly. Gently press the dough together to seal the seam. Tuck the ends under the coil and fold into the loaf pan. Repeat with the second portion of dough and place into second loaf pan.
Cover with floured kitchen towel or paper towel and let rest for 1.5 - 2 hours for the second proof. Optionally, after 1.5 hours of proofing at room temperature, place in refrigerator to finish proofing overnight. The dough should rise above the lip of the loaf pan and should not spring back when pressed
Preheat oven to 375 degrees F while dough is proofing
Whisk together egg and milk or water to create an egg wash. Brush over the tops of the loaves using a pastry brush
Place loaves in middle rack of the oven and bake for 40 minutes until golden brown. They will sound hollow when you tap them with a spoon
Let cool for 5-10 minutes before sliding out of the loaf pans. Let cool for another 10 - 20 minutes on a wire rack before slicing and serving