In a large mixing bowl combine flour, sugar, cocoa powder, baking powder, and salt.
In a separate large mixing bowl, add the buttermilk, mayonnaise, eggs, and vanilla. Mix well to combine.
Stir the mayonnaise mixture and the coffee into the flour mixture until combined. The batter will be thin. Scrape batter into prepared pan and smooth top. Bake until a cake tester comes out with just a few crumbs attached, 30-40 minutes.
Cool cake in the pan on a wire rack for then minutes before turning out. Remove cake from pan and allow to cool completely.
For The Ganache Icing
Place the chopped chocolate into a medium-sized, heatproof bowl.
In a small saucepan, heat the cream until it just begins to simmer and then pour it over the chopped chocolate. Let it sit a minute and then gently stir with a wooden spoon until the chocolate has fully melted and the ganache is smooth.
Then cover and set aside until it is set enough to drizzle over the cake, but isn't too runny.
Notes
It’s very important to allow your cake to cool. If you move it while it’s still warm it is likely to fall apart.
Always use full fat mayonnaise, any other mayonnaise will not give you the desired results.
Try to use the best quality cocoa powder you can find. It really makes a huge difference to the flavor of your cake.