Preheat the oven to 180 C / Gas 4. Grease and line two 23cm cake tins with baking parchment.
Beat together the margarine and sugar until light and fluffy. Add the eggs, one at a time and beat until well mixed then add the vanilla extract. Sift in the flour, bicarb and salt and mix well. Transfer to the prepared cake tins.
Bake in the preheated oven for 30 to 40 minutes or until a skewer inserted into the centre comes out clean. Remove from the oven and cool completely.
For the icing:
Beat together the margarine and chocolate then gradually mix in the sugar, water and vanilla extract. Beat until fluffy.
To assemble the cake, slice a little off the top of what will be the lower cake layer to provide an even base for the second layer, if necessary. Place onto a serving plate. Spread over half of the icing then sandwich the second layer on top, again carefully slicing off any excess sponge to make a nice level top. Spread the remaining icing on top and around the sides if there is any icing left over. Slice and enjoy.