1 Preheat oven to 350 degrees F (175 degrees C). Sift together the flour, sugar, cocoa, baking soda and salt. Sift once more, then sift again into a 9 inch square pan.
2 Make a well in the center, and measure in the oil, vinegar and vanilla. Pour water over all and mix with a fork until smooth.
3 Bake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.
4 To make the frosting: In a saucepan, combine chocolate, margarine and water. Cook over low heat until melted and smooth. Remove from heat and stir in vanilla and confectioners’ sugar. Fold in miniature marshmallows. Spread over cake.