Step 1

Heat the oven to 350°F with a rack in the middle position. Mist a 9-by-5-inch loaf pan with cooking spray, then line it with an 8-by-14-inch piece of kitchen parchment, allowing the excess to overhang the long sides of the pan.

Step 2

In a medium bowl, whisk together the flour, the 42 grams (⅓ cup) cocoa, the baking powder, baking soda, 1 teaspoon cinnamon, the cayenne and the salt. Add the chopped chocolate and toss to coat.

Step 3

In a large bowl, whisk the eggs. Add the brown sugar, oil, 135 grams (½ cup) of the sour cream, 110 grams (⅓ cup) of the tahini and 1 teaspoon vanilla; whisk until well combined. Add the flour mixture and mix with a silicone spatula until just combined. Scrape the batter into the prepared pan and smooth the surface, then rap the pan on the counter a few times to remove any air bubbles. Bake until a toothpick inserted at the center of the loaf comes with a few moist crumbs attached, 55 to 60 minutes.

Step 4

Cool in the pan on a wire rack for about 15 minutes. Lift the loaf out of the pan using the parchment and set it directly on the rack. Let cool completely.

Step 5

Remove the parchment liner under the loaf. In a medium bowl, combine the powdered sugar, the remaining ¼ teaspoon cinnamon, the remaining 2 tablespoons cocoa, the remaining (68 grams) ¼ cup sour cream, the remaining 3 tablespoons tahini and the 1 remaining teaspoon vanilla. Whisk until smooth. Pour the glaze onto the loaf and smooth with a spatula, then sprinkle with additional cayenne. Let stand for about 15 minutes before slicing.