Combine first five ingredients in a large pan. Stir over low heat until sugar dissolves. Boil, then immediately simmer uncovered for 5 minute Transfer to a large bowl and cool.
Add sifted flour and eggs, and beat with electric mixer until smooth. Pour int dish.
Bake in mod (160c ff) oven for 50 minute Cool for 10 min then turn onto rack.
Frosting: combine butter water and sugar in small pan, stir over heat without boiling until sugar dissolves. Sift icing sugar and cocoa into medium bowl. Stir in hot butter mixture in batches. Cover and refrigerate for 1 hours Beat until spreadable.