Yes, you can use margarine instead of butter. Use a margarine that is high in vegetable fats (usually sold in wrappers) as opposed to high in water and gelatine (usually sold in tubs and cups). And avoid baking margarine, which has a less pleasant taste when eaten raw.I'm not familiar with AH planten margarine though. Should be fine I guess.
For more complicated recipies Google 'vegan buttercream'.
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