Beat the margarine and sugar until light and fluffy.
Sieve the flour, cocoa powder and salt into a bowl. Mix the food colouring and vanilla into the buttermilk and mix well to combine.
Add 2 eggs to the sugar/margarine mix together with a few tablespoons of your dry ingredients and beat until well combined. Add the last 2 eggs and keep mixing until fully incorporated.
Add half of the dry ingredients and mix together until combined, then add all of the buttermilk mixture and the remaining dry ingredients, again mixing until combined.
In a small bowl add the vinegar and bicarbonate of soda, stir well then add to the cake mix and stir to incorporate.
Pour the mixture into an 8-inch half-sphere cake tin. Bake in the preheated oven for 40-45 minutes until an inserted skewer comes out clean.
Remove from the oven, allow to cool for 5-10 minutes and then turn out and allow to cool completely.
To Make the Cream Cheese Frosting:
Beat the butter and cream cheese together until light and fluffy.
Add half of the icing sugar and vanilla and beat slowly until combined then add the remaining icing sugar mix in.
To Decorate the Cake:
Level off the bottom of the half-sphere sponge and then cut in half lengthways to form the two brain hemispheres.
Fill between the two halves with the frosting and gently put it back together. Cover the sponge with a thin layer of frosting, then cut a thin slice off each side to form a more brain-like shape. Lightly frost the cut sides.
Refrigerate the frosted sponge for 30-60 minutes to set the frosting.
Roll out thin sausages of icing and form a concertina to give the brain a surface pattern.
To Serve:
Just before serving spoon over the edible fake blood for that fresh wet look.
Notes
For a touch of added realism, place some crumpled tissues and a knife dipped in the edible fake blood on the cake stand.