Combine the flour, sugar, yeast and salt in the bowl of a stand mixer with the paddle attached.
Add warm milk and stir until well mixed, then add the egg and half a yolk and continue mixing until blended. You can do this by hand or use the paddle attachment of your stand mixer. Add the softened butter a chunk or two at a time and continue mixing, scraping down sides of bowl with a scraper. If the butter is not fully integrated, that is okay. The dough hook can finish the job!
Attach the dough hook to the stand mixer and let the dough hook knead for about 5 minutes. The dough will be sticky and might not completely leave the sides 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 it should have some snap and elasticity to it.
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). 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.
Press and roll or pat dough it into a 16x11 inch rectangle. The 16 inch side should be on the bottom closest to you. If the texture of the dough is right, you should be able to do this easily without even using a rolling pin. You can just pat it.
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 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 the chocolate mixture all over the rectangle as evenly as you can.
Working from the 16 inch side, loosen the edge with a bench scraper or thin spatula and roll upward into a tight 16 inch jellyroll and pinch edges to seal. Keep using the 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. This is to set the chocolate and make the log easier to work with.
While the dough is in the freezer, grease two 5x3 inch loaf pans or one 8 1/2 by 4 1/2 inch loaf pan and line with strips of parchment.
Remove log 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. Repeat with remaining half of the jellyroll so that you have two twists.
If you are making an 8 1/2 by 4 1/2 inch loaf, lay the two twists side by side lengthwise in the pan. If you are making two mini loaves, cut each of your twists cross-wise so that you have four small, stubby twists. Arrange the smaller twists in the loaf pan, squishing to fit.
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.