Cake: Preheat an oven to 180 degrees C. Grease and flour three 25cm round cake tins.
Cream the margarine, sugar, eggs and vanilla essence in a large bowl. Make a paste of cocoa and food colouring in a small bowl then add to margarine mixture.
Pour in the flour alternately with the buttermilk, mixing until just incorporated. Mix the salt, bicarb soda and vinegar in a small bowl, then while fizzing fold into the batter, mixing just enough to evenly combine.
Pour the batter into prepared cake tins. Bake in the preheated oven until a skewer inserted into the centre comes out clean; 20 to 25 minutes. Cool in the cake tins for 10 minutes before removing to cool completely on a wire rack.
Icing: Cook 3 tablespoons of flour with the salt and milk in a frypan over low heat, stirring constantly until thick. Let cool completely.
Cream butter, sugar and margarine in a separate bowl. Stir in 2 teaspoons of vanilla essence. Then add flour mixture to bowl and cream together.
To Assemble: Stack the cake layers with the icing as a filling between, use remaining icing to coat the whole cake.