Grease a deep 19cm square cake pan, line base with paper and grease the paper as well.
Melt margarine in a pan and add chocolate,stir over low heat until chocolate is melted.
Cool for five minutes.
Beat butter, essence and sugar in a small bowl with an electric mixer until light and fluffy.
Beat in eggs one at a time.
Transfer mixture to a large bowl.
Stir in sifted flour, then chocolate mixture, nuts and cream.
Spread mixture into prepared pan.
Bake in moderate oven for about 45 minutes.
Cool in the pan slightly. then move to cooling rack.
Spread with chocolate icing.
Cut when set.
For Chocolate Frosting:Melt chocolate and butter over simmering water. Cool to room temperature.Beat with wooden spoon until thick and spread able.
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