• Make the bread dough.
• In the bowl of your stand mixer, place the flour, cream of tartar, instant yeast and sugar, and use a handheld whisk to combine well. Add the salt and whisk to combine well. Add the milk, eggs and butter, and mix on low speed with the dough hook until combined. Raise the mixer speed to medium and knead for about 5 minutes.
• The dough is a lovely, smooth, enriched dough. It climbs up the dough hook during kneading but remains intact and smooth. Spray a silicone spatula lightly with cooking oil spray, and scrape down the sides of the bowl.
• Transfer the dough to a lightly oiled bowl or proofing bucket large enough for the dough to rise to double its size, spray the top of the dough with cooking oil spray, and cover with an oiled piece of plastic wrap (or the oiled top to your proofing bucket).
• Place the dough in the refrigerator for at least 12 hours and up to 3 days.

Prepare the dough for shaping.

• On baking day, grease a standard loaf pan (approximately 9-inches x 5-inches) and set it aside.
• Turn out the chilled dough onto a lightly floured surface and smooth out the dough by scraping it up with a lightly floured bench scraper or cake turner, and turning it over on itself. Be sure that you are using a very light touch, sprinkling the dough with more flour and kneading it lightly. Repeat scraping and folding until the dough has become smoother. Do not overwork the dough or you will incorporate too much flour and it will not rise properly.

Shape and fill the dough.

• Divide the dough into two equal pieces. Set one piece aside and cover lightly with a tea towel so that it does not dry out.
• With a floured French rolling pin, roll the remaining half of dough flat into a rectangle until it is about 6-inches x 10-inches, shifting the dough around frequently and sprinkling lightly with more flour as necessary to prevent sticking.
• Brush the entire surface of the dough with about half of the melted butter from the filling. Sprinkle with half of the grated chocolate, and then half of the cinnamon sugar. Press gently on top to secure the chocolate and cinnamon sugar.
• Using a very sharp knife or pizza wheel (the wheel works best), slice the rectangle into 3 strips along the 10-inch length. Carefully lift the strips and stack them squarely on top of one another.
• Slice through all 3 strips in 5 places, making 5 equal stacks of small rectangles, each 3 high. Set the stacks aside.
• Gather together any of the filling that has fallen off, and set it aside.
• Repeat with the remaining piece of dough and the remaining filling in the exact same manner.

Assemble and bake.

• Place all of the stacks of dough in the prepared loaf pan, all back to back. The pieces of dough will touch, but will reach across the length of the loaf pan without being too crowded. Sprinkle the top with any filling that fell off during shaping.
• Cover the dough loosely but securely with an oiled piece of plastic wrap, and set in a warm, draft-free location to rise until doubled in size (about 1 1/2 hours, but rising time can vary greatly depending upon your kitchen environment).
• About 20 minutes before your dough has finished its final rise, preheat your oven to 350°F.
• Once the dough has finished rising, remove the plastic wrap and place the pan in the center of the preheated oven.
• Bake until deep golden brown all over (35 to 40 minutes), or until the top of the bread is relatively firm when pressed gently on top.
• Allow to cool for about 10 minutes in the pan before running a thin spatula around the edge of the loaf pan and turning out the bread onto a wire rack to finish cooling until no longer hot to the touch. Serve immediately.