Preheat the oven to 180°C and prepare two cake tins by greasing them.
In a large bowl, combine the flour, black cocoa, sugar, baking powder, baking soda, and salt.
Add the eggs, buttermilk, oil, vanilla, and coffee to the dry ingredients and whisk until the mixture is smooth.
Divide the batter evenly between the prepared cake tins and bake for 30 to 35 minutes.
While the cakes are baking, prepare the blackberry compote by cooking the blackberries, sugar, lemon juice, and cornstarch together until the mixture thickens and becomes jam-like.
Once the cakes are done, whip together the powdered sugar, cream, and black cocoa until smooth.
To assemble, place one layer of the cake on a cake board and add the second layer on top.
