Combine the flour, sugar, yeast and salt in the bowl of a stand mixer with the paddle attached.
Add milk and stir until well mixed, then add the eggs, yolk and vanilla and continue mixing until blended. Add the butter a chunk or two at a time and continue mixing, scraping down sides of bowl with a scraper.
Attach the dough hook to the stand mixer and let the dough hook knead for about 5 minutes. The dough will be sticky and should not completely leave the sides of the bowl as it is being kneaded. To make sure the dough feels right, stop the kneading occasionally and push dough off the side of the bowl and into the center with a scraper. It will be a sticky dough, but kneading it should give it some elasticity. If it is incredibly sticky and not leaving the side of the bowl as it is kneaded, dust sides of the bowl with a little flour.
Scrape the dough into a bowl that has been greased very well with oil and roll the dough around so that it’s slick. Cover with plastic wrap and let rise in a warm place until doubled in bulk (about 40 minutes to an hour with the fast rising yeast). You should have 2 pounds 3 oz (35 oz) of dough. It should be a slick and oily dough. While the dough is rising, you may want to go ahead and get your egg yolk/milk mixture ready and prep your chocolate filling ingredients.
Once the dough has risen, lay out a big pastry mat. You shouldn't need to flour the mat because the dough should be oily and not at all sticky. If the dough is sticky for some reason, you can use a little flour. Do what you need to do to control that dough!
Take half of the dough (17 1/2 oz) and put it on the pastry mat. Press and roll or pat it into a 16x11 inch rectangle. The 16 inch side should be on the bottom closest to you.
Prepare filling. Melt the chocolate and butter together in a microwave-safe bowl (I use a 2 cup Pyrex) on high stirring every 30 seconds. When completely melted, stir in the sugar.
Now you are going to spread half the chocolate mixture across your rectangle, but before you do brush a little of the egg yolk/milk mixture along the 16 inch side of the rectangle.
Now spread half of the chocolate mixture all over the rectangle as evenly as you can.
Working from the 16 inch side, roll upward into a tight 16 inch jellyroll and pinch edges to seal. You can use a bench scraper or thin spatula to loosen dough from mat and push upward as you roll. Now you should have a messy 16 (ish) inch jellyroll.
Carefully transfer the jellyroll to a piece of parchment and then to the freezer to set the chocolate. Don't worry too much if the log gets a little messed up during the moving process, just get it in the freezer. Repeat with the remaining dough and chocolate so that you have two jellyrolls. Let them both sit in the freezer for about 15 minutes.
While the dough is in the freezer, grease four 5x3 inch loaf pans (or if making 2 loaves, use two 8 1/2 x 4 1/2 inch loaf pans), line with strips of parchment and dust with flour.
Remove one of your jellyrolls from the freezer and set it on the pastry mat. At this point it will be slightly firm and cold but still malleable. With your hands, gently "neaten it" a bit so it's 16 inch cylinder. Cut in half crosswise so you have two 8 inch pieces. Take one of the pieces and cut lengthwise, exposing chocolate. Put the split pieces on their backs cut side up. Pinch the tops of the split pieces together and bring one strand over the other, alternating each time, until you have a twist. Squish the twist into a 3x5 inch loaf pan. Repeat with remaining half of the jellyroll plus the one you have in the freeze to make a total of four loaves.
If you are making two 8 1/2 by 4 1/2 inch loaves rather than four, the method will be similar. Take one of your jellyrolls out of the freezer, spiff it up to make it a neat cylinder, then cut it in half cross-wise to make two 8 inch pieces. Take one piece and split it lengthwise, then twist the halves together. Set lengthwise in the 8 1/2 by 4 1/2 inch pan. Repeat with remaining 8 inch piece and lay your new twist adjacent to the other one in the pan. Squish ends of the two twists together and taper a little. Take the second 16 inch jellyroll out of the freezer and repeat the process so that you have two loaves total. Cover loosely with greased plastic wrap and let rise for about 40 minutes (or 70 minutes if you used active dry and not fast rising).
Preheat oven to 350 degrees F. Brush tops with remaining egg wash and sprinkle with sparkling sugar if desired. Bake for about 40-45 minutes or until a thermometer inserted reads 205 degrees F.
Let cool completely (or let cool and freeze), then slice and reheat just slightly.