I found this recipe online, but have not tried it yet.
Chocolate BreadOne 9-inch (23 cm) loaf
Use very good cocoa powder; here I used Valrhona, since I think the darker color and strong taste are important in the loaf. But another brand of Dutch-process cocoa should work fine, too.
If you don't have the coffee powder, you can leave it out. I like it since it add a subtle boost to the chocolate flavor. This recipe works best with bread flour, but if you can't get it, all-purpose flour works well, too. I've made a few notes at the end of the recipe about yeast and flour substitutions.
3/4 cup (180 ml) whole or low-fat milk, heated until just tepid
1 envelope active dry yeast (1/4 ounce, or 2 1/4 teaspoons)—see Note
6 tablespoons (75 g) sugar
4 tablespoons (55 g) butter, salted or unsalted
3 ounces (85 g) bittersweet or semisweet chocolate, coarsely chopped
1 1/2 teaspoon instant coffee or espresso powder (optional)
1 large egg
1/2 teaspoon vanilla extract
3/4 teaspoon sea salt
2 cups (280 g) bread flour
1/4 cup unsweetened Dutch-process cocoa powder
3/4 cup (3 1/2 ounces, 90 g) chocolate chips or coarsely chopped bittersweet or semisweet chocolate
1/2 cup (70 g) toasted pecans, walnuts, almonds, or hazelnuts, coarsely chopped (optional)
1. In the bowl of a stand mixer or in a large bowl, sprinkle the yeast over the milk. Add one tablespoon (11 g) sugar, then set aside in a warm place for 10 to 15 minutes, until bubbles form on the surface.
2. While the yeast is activating, in a small saucepan, melt the butter and 3 ounces (85 g) chocolate over a pan of barely simmering water. Stir occasionally, until the chocolate is melted and the mixture is smooth. Remove from heat.
3. Once the yeast mixture is frothy, mix in the remaining sugar, the instant coffee (if using), the egg, vanilla, and sea salt.
4. Stir in half the flour and cocoa powder, then the melted butter and chocolate, then the remaining flour mixture, stirring until well-incorporated. If using a stand mixer, attach the dough hook and beat for five minutes, until smooth. If making by hand, mix vigorously with a flexible spatula for the same amount of time. The dough will seem quite moist, resembling sticky brownie batter when ready.
5. Cover the bowl and let rise in a warm place for 2 hours.
6. Butter a 9-inch (23 cm) loaf pan.
7. Stir in the chopped chocolate and nuts, if using. Then use a spatula to fold the dough over on itself in the bowl for about thirty seconds, then transfer it to the buttered pan, pressing a bit to spread it to the corners. Let rise in a warm place for one hour.
8. Ten minutes before you're ready to bake the bread, preheat the oven to 350ºF (175ºC.)
9. Bake the bread for 35 to 40 minutes, until it feels done and sounds hollow when you tap it. You can stick an instant-read thermometer in the bottom if you're unsure; the bread is done when the temperature reads 180ºF (82Cº).
