Prepare the dough: In a small cup, stir together the yeast and the warmed milk. Set aside for 5 minutes (it should become a little frothy. If it doesn’t, your yeast might be dead).
In a small bowl, whisk together the egg and the sugar, then add the yeast mixture and stir it in well. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, stir together the flour and the salt.
Add the egg mixture to the mixer bowl and beat it in until it is almost completely incorporated, then switch to the dough hook.
Add the butter to the bowl, and knead it in with the dough hook for about 6 minutes or so. The dough should feel slightly tacky when touched but the mixer bowl should be clean with no dry bits.
Transfer the dough to a greased bowl and cover with plastic wrap and let rise til doubled in size (this take about an hour to 1.5 hours).
Roll the dough and prepare the filling:
When the dough has doubled, punch it down then transfer it to a floured surface and roll it to a 13×22 inch rectangle. Let it rest while you prepare the filling.
Make the filling: use a mini food processor to pulse the chocolate and cinnamon until it is powdery with small chunks (the size of mini chocolate chips). Transfer it to a bowl and work in the butter with your hands.
Dump the chocolate filling onto the dough and spread it out thinly and evenly so every inch is pretty much covered.
Roll the dough into a 12-inch long, as tightly as possible, then trim the ends off (you don’t have to but I think it’s neater this way) and cut the log into 12 pieces.
Grease two muffin pans (or one 12-muffin cup pan) and place the rounds of dough in the greased pans. Cover loosely with plastic wrap and let rise again for about 45 minutes until the buns puff a bit.
Prepare the egg wash & streusel (if using).
Prepare the egg wash by mixing the egg with the tablespoon of milk. Brush the buns lightly with the egg wash.
If you want streusel topping, combine the flour, sugar and salt in a bowl and work in the butter with your fingertips. Then top each egg washed bun with a mound of streusel before baking.
Bake the buns in an oven preheated to 350°F (175 °C) for about 30 minutes, until they are a deep brown and spring back when touched.
Let cool before serving (if you can wait that long).