Preheat the oven to 180℃, line two 5 inch tins with baking paper and brush with oil, set aside.
In a mixing bowl add the oil, mayonnaise, egg and whisk. Add the white sugar, vanilla and whisk. Add the milk mixture and whisk.
Sift in half the dry ingredients (flour, corn starch, baking powder, baking soda, salt) and mix until partially combined, add the hot water and mix until smooth. Add the remaining dry ingredients and mix until combined.
Divide the batter between the tins and bake for 25 minutes until risen or until a tooth pick inserted into the center comes out clean.
While this is happening let us make the frosting by adding all the ingredients into a heavy bottom sauce pan and placing on low heat.
Stir well, once the chocolate is melted and the mixture is smooth take it off the heat (do not over cook it) and let it cool completely (around 1 hour), one cooled, whip the frosting for 2-5 minutes with a whisk or electric hand beater until pale and fluffy.
Once the cakes are done remove from the tin, level them by slicing the dome off with a knife, wrap in cling wrap and let it cool. After the cake is cooled place one cake on a plate and top with the frosting that should now be thick and spreadable.
Spread the frosting evenly over the cake and place the next cake on top. Frost the top of the cake and he sides in a rustic swirl like manner. Slice and serve or just bite into it directly.