Sift together the cake flour 2 tbsp cocoa, salt and charcoal and set aside.
In a separate bowl, mix together the buttermilk, vanilla, baking soda and eggs.
Add the vinegar and stir. It may fizz a little - set aside.
Cream together the shortening and sugar until fluffy.
Alternate adding the flour mixture and the buttermilk/egg mixture until all combined.
Add the remaining cocoa and beat until combined.
Pour the batter into well-greased 35 x 45cm sheet cake pan. I didn't have one, so I split the mix between two smaller trays.
Use a large offset spatula or knife to even out the surface.
Bake for 20 minutes. Remove the pan from the oven and allow the cake to cool completely before icing.
For the classic velvet frosting
Combine the milk and flour in a saucepan over medium heat.
Whisk constantly until thick. Remove from the heat and let cool to room temperature.
Cream the butter and sugar until light and fluffy.
Add the cooled milk mixture and vanilla and beat very well. It may look separated, which means you have to keep beating until fully combined. It should be fluffy and light, similar to whipped cream.