The deep, rich flavors of dark chocolate mirror the intense and often somber themes Holly encounters in her investigations, perfect for a contemplative dessert.
Preheat oven to 350°F (175°C).
Grease and flour an 8-inch round cake pan.
In a bowl, whisk together flour, sugar, cocoa, baking powder, baking soda, and salt.
Add buttermilk, oil, egg, and vanilla, mix until combined.
Stir in boiling water until smooth.
Pour into prepared pan and bake for 25 minutes.
Cool before serving.