Chocolate Swirl Brioche

Set theindulgent mood of a lazy summer day with sliced brioche swirled with semisweet chocolate.

1 cup (2 sticks) unsalted butter, cut into Tbsp., softened, + more for bowl and pans
2 Tbsp. warm water (105°F-110°F)
¼ cup + 1 tsp. sugar
2 tsp. active dry yeast
4-4¼ cups all-purpose flour, unbleached
2¼ tsp. sea salt
9 large eggs
9 oz semi-sweet chocolate, finely chopped (about 1½ cups)
coarse sanding sugar, for sprinkling

Preheat oven to 375°F (190°C/170°C fan, Gas Mark 5). Butter two 8½x4½x2½" loaf pans; set aside. 

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

Put 4 cups flour, remaining ¼ 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.

Add butter, 1 Tbsp. 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 ½ 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 1½ hours. Punch down dough; cover with plastic wrap, and refrigerate overnight (at least 12 hours).Transfer dough to a lightly floured work surface. Divide dough in half. Roll out each half into a 7x15" 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.

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.

Makes 2 loaves.