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I have been asked to make a 3 tier wedding cake, rustic style, bottom tier fruit and top two tiers sponge. Is royal icing OK to use on the two sponge cakes to create the rustic effect as most cakes shown use buttercream and I can’t use that on the fruit cake .

Hi Rosemary

As long as you do a thin buttercream crumbcoat first and cover the sponge cake with a layer of marzipan , royal icing can be used. It doesn’t have to be the brittle type. If you add a teaspoon of glycerine to 450g of icing, it will remain softer to cut and nicer to eat. For a nice firm base, allow marzipan covering to dry for a couple of days before applying royal icing. This will seal the oils in the marzipan and prevent it from discoluring and weaking the icing.
Alternatively, you could use white chocolate ganache to make your rustic design. Fruit cake and ganache go well together, although the shelf life of the cake will be shortened. Ganached fruit cake can be stored in the fridge to extend the ganache life.