How to Make It:
Step 1: Grease well with unsalted butter a loaf pan that is no more than 9 inches x 5 inches. Set it aside.
Step 2: In a medium-sized microwave-safe bowl, place the butter and chopped chocolate. Microwave at 30-second intervals on high power, stirring in between, until the chocolate and butter are smooth and shiny. Set it aside to cool slightly.
Step 3: In the bowl of a stand mixer fitted with the paddle attachment, mix the flour, salt, cocoa powder, cinnamon, baking soda, cream of tartar and sugar on low speed until well-combined. Add the yeast and mix once more on low until well combined. Add the chocolate chips, and mix on low until just mixed.
Step 4: To the stand mixer bowl, add the melted chocolate, and then the oil, vanilla and eggs, and mix on low until the dry ingredients are just moistened. With the mixer still on low speed, add the milk in a slow but steady stream. Once the dough has become together, turn the mixer on medium to high, and mix for about 4 or 5-minutes.
Step 5: Scrape the dough into the prepared loaf pan, spread until it is evenly distributed in the pan, and smooth the top with wet fingers.
Step 6: Cover the dough loosely with plastic wrap and leave to rise in a warm, humid place until the dough has risen about 3/4-inch above the lip of the loaf pan. Depending upon the environment in your kitchen, this may take more or less than 40 minutes.
Step 7: Preheat your oven to 375 degrees F.
Step 8: Once the dough has risen, remove the plastic wrap and place in the center of the preheated oven. Bake, rotating once during baking, until the bread has developed a nice crust, and sounds hollow when you thump it with your fingers (about 45-minutes).
Step 9: Remove from the oven, and cool for about 10-minutes in the pan before placing it on a wire rack to cool completely. Cool completely before slicing and serving.