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.

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.

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.

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.

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.