In the bowl of your electric mixer, stir together the warm milk, 2 teaspoons sugar and yeast. Let sit until foamy, about 5 minutes. With the paddle attachment on the mixer, beat in 1 cup of flour until combined. Add in the eggs, yolk, vanilla extract, salt and remaining 1/2 cup sugar. Beat on medium-low speed until combined. Gradually add in the remaining flour 1/2 cup at a time, beating on medium-low speed. Once a sticky dough has formed, add the butter pieces in a few handfuls at a time. Beat for 4 to 5 minutes, until the dough is shiny and stringy and soft.
Place the soft and sticky dough in a well oiled bowl and cover with a towel. Let rise in a warm place for 1 1/2 to 2 hours.
Remove the dough from the bowl and place it on a floured workspace. Whisk together the egg and heavy cream. Roll the dough out into a large rectangle (mine was about 12×20) and brush the edges with the egg wash. Gently spread the 6 tablespoons softened butter all over the dough – try not to press too hard and tear the dough, just gently rub it on with your hands. Add the chopped chocolate, the cinnamon and the sugar. Starting at the bottom, roll the dough tightly up into one large log as if you were about to make cinnamon rolls.
Preheat the oven with 350 degrees F. Rub two muffin tins with softened butter.
Slice the dough into 24 somewhat equal rounds (they will look like cinnamon rolls!) and place each round in the muffin tin. Cover the muffin tins with a towel and let rise for another 30 to 60 minutes. Brush the tops with the remaining egg wash.
Bake the breads for 25 to 30 minutes, or until the tops are golden and set. Let cool slightly before removing from the tin. Serve immediately. I have stored these once they are completely cool – in a resealable bag then warmed them for a few seconds before serving.