adapted from Baked NYC; makes one pan of mini loaves (8/pan), or one 9- by 5-inch loaf, or 12 cupcakes
Preheat oven to 350°F. Grease desired pan with spray or shortening (I use canola oil spray).
Mix together dry ingredients (cocoa, sugars, flour, leavenings, and salt) in a large bowl or electric mixer.
Whisk together the buttermilk, oil, eggs and additional yolk, and vanilla extract. Add to dry ingredients and mix until combined. Stir in the chopped bittersweet chocolate.
Portion batter into desired pan. Bake pan of mini loaves about 30 minutes, big loaf 50–60 minutes, and cupcakes about 20 minutes. Cool in pan for 10 minutes, then remove to rack to cool completely.