1/2 cup milk
In a glass bowl, heat the milk and butter in the microwave until butter is melted.
In the bowl of an electric mixer fitted with the paddle attachment, pour yeast over warm water.
Mix the orange zest into the sugar, using your fingers to release the oils and fully incorporate the zest. Beat the sugar, vanilla, and eggs into the warm water and yeast until blended well. Add the salt, milk mixture and 1 cup of flour and mix well.
Add the remaining flour a 1/2 cup at a time until all the flour has been added and mixture is fully combined. Place dough in a greased bowl and cover. Allow dough to rise until doubled in size, about 2 1/2 hours.
Once dough has doubled, punch down and place on a floured board. Allow dough to rest for 10 minutes, then rollout out into a rough 8 x 14 rectangle. Cover surface with chocolate chunks and roll into a log, starting with the short end. PLace dough seam side down in a well greased loaf pan. Allow dough to rise for 1 1/2 hours.
Preheat oven to 350 F. Brush top of loaf with egg wash of 1 egg and 1 tablespoon milk. Bake for 45 minutes until golden. Allow to cool completely. Store bread well wrapped at room temperature for up to 4 days.