Preheat oven to 180 degrees C. Sift together the flour, sugar, cocoa, bicarb soda and salt. Sift once more, then sift again into a 23cm square pan.
Make a well in the centre, and measure in the oil, vinegar and vanilla. Pour water over all and mix with a fork until smooth.
Bake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.
To make the icing: In a saucepan, combine chocolate, margarine and water. Cook over low heat until melted and smooth. Remove from heat and stir in vanilla and icing sugar. Fold in miniature marshmallows. Spread over cake.