Preheat the oven to 180 degrees Celsius, line and grease a and sift the dry ingredients together in a large bowl. Normally I think sifting is heinously cumbersome but it truly is necessary with cocoa. Ottolenghi suggests, in his book Sweet, to sieve cocoa together with sugar or something else, to stop it from sticking to itself. Just thought you might like to know.
In a small bowl, add the vinegar to the milk, to make buttermilk. This will help give the cake oompf.
Add all the wet ingredients to the dry, and use a whisk to thoroughly combine them, until no pesky cocoa lumps remain. Pour the mixture into the greased and lined tin, and cook for 30-40 minutes, or until a skewer comes out mostly clean. I found all of my trials needed close to the 40 minute mark, but our ovens may (/absolutely do) differ.
While the cake is cooking, it's time to make your icing of choice. Whichever you choose (buttercream) use a hand mixer to beat the ingredients until they magically become a thick, smooth, icing consistency. I like to both add salt and top with salt - it makes the chocolate flavour more intense, and disguises some of the olive oil taste in the dairy free option.
When the cake is ready, remove it from the oven and allow it to cool completely before icing, so the icing doesn't melt. Spread the icing liberally atop, sprinkle with sea salt flakes, and pass a slice to Aunty Sheryl.