Preheat oven to 180 C / Gas 4. Grease and flour two 23cm (9 in) cake tins. Sift together the flour, cocoa, bicarbonate of soda and salt. Set aside.
In a large bowl, cream together the butter or margarine and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the milk, mixing just until incorporated. Pour cake mixture into prepared tins.
Bake in the preheated oven for 40 to 45 minutes, or until a skewer inserted into the centre of the cake comes out clean. Allow to cool.
Beat the butter by hand with a wooden spoon or with a mixer on medium speed until creamy.
Add icing sugar, cocoa powder, heavy cream, salt, and vanilla extract and blrn with the wooden spoon or with the mixer running on low. (Add more icing sugar if frosting is too thin or another Tablespoon of cream if frosting is too thick.
Once caked have cooled spread a layer of buttercream on top of one of the caked, the sandwich the second cake on top. Spread the rest of the buttercream to cover the whole of the cake. slice to serve.