Preheat oven to 180 degrees C. Grease and flour one 30x20cm cake tin.
Cake: Sift the flour, white sugar, baking powder and salt together. Mix in the margarine, milk and vanilla. Beat until well combined. Add the egg whites and beat for 2 minutes. Pour the batter into the prepared pan.
Bake in the preheated oven for 20 to 25 minutes. Let cool then ice.
Icing: Melt butter in a saucepan until it begins to brown. Remove from heat and add the icing sugar, evaporated milk, hot water and 1 1/2 teaspoons vanilla essence. Beat at medium speed for three minutes. Then pour icing over cake and allow icing to set.