1. Butter a large bowl; set aside. Stir together warm water, 1 teaspoon sugar, and the yeast in a small bowl. Let stand until foamy, about 5 minutes.

2. Put 4 cups flour, remaining 1/4 cup sugar, and the salt into the bowl of an electric mixer fitted with the paddle attachment. Add yeast mixture; mix on medium speed until combined. Add 8 eggs, 1 at a time, mixing well after each addition; mix until well combined, about 2 minutes. Replace paddle with dough hook. Mix on medium speed 2 minutes.

3. Add butter, 1 tablespoon at a time, mixing until combined after each addition. Mix until dough is smooth, shiny, and elastic, about 10 minutes. If dough is very sticky and wet, add up to 1/2 cup flour (only the bottom quarter of the dough should stick to the bowl); mix until combined. Transfer dough to buttered bowl. Cover with a clean kitchen towel, and let rise at room temperature until almost doubled in bulk, about 1 1/2 hours. Punch down dough; cover with plastic wrap, and refrigerate overnight (at least 12 hours).

4. Butter two 8 1/2-by-4 1/2-by-2 1/2-inch loaf pans; set aside. Transfer dough to a lightly floured work surface. Divide dough in half. Roll out each half into a 7-by-15-inch rectangle. Sprinkle chocolate over rectangles. Starting from a short side, tightly roll each into a log; pinch seam to seal. Place each log, seam down, in a buttered pan. Cover with a clean kitchen towel; let rise at room temperature until dough reaches rim of pans and no longer feels cool, about 3 hours.

5. Preheat oven to 375 degrees. Lightly beat remaining egg; brush tops of loaves with egg. Sprinkle with sanding sugar. Bake until tops are deep golden brown, about 35 minutes (cover with foil after 20 minutes if browning too quickly). Run a knife around edges of pans to loosen, and turn out onto a wire rack; let cool completely.