Start out by preheating your oven to 350ºF, grease and line your cake tins. Even if you're using silicon cake pans, I'd recommend lining the base with parchment paper. This is an extremely light and delicate cake and prone to sticking to the bottom.
In a large mixing bowl, cream together your sugar and butter until light and fluffy.
1 cup unsalted butter, 2 cups superfine granulated sugar
Next add the vanilla and eggs one at a time. Mix until just combined into a smooth batter.
2 teaspoon vanilla extract, 2 large eggs
In a small bowl, mix together the cocoa powder and vinegar. This will create a paste and help to activate the cocoa - making this deeper in color and richer in taste. Add this paste to the cake along with the buttermilk.
1 tablespoon distilled white vinegar, 1 cup buttermilk, ¾ cup black cocoa powder
Sift the dry ingredients into the wet mixture. This should be the flour, cornstarch, baking soda and salt. Mix this until just combined.
1½ cup all purpose flour, ¼ cup cornstarch, 1½ teaspoon baking soda, ½ teaspoon salt
Slowly add in the warm water, stirring until combined and smooth.
½ cup water
Pour the batter into your prepared cake tins and bake in the center of your pre-heated oven until a toothpick comes out clean.
Remove from the oven and allow to cool until you are able to handle the pan. Gently remove the cake from the pans - any bits that fall apart can always be stuck together with some of the handy frostings!