Hi MS,
Depends how big your cake is. But for a 9" (we're metric here - so that's more or less) round cake, you use 2+ cups of icing mixture (you know the deal... add a little if you need to), 2 teaspoons of cocoa powder**, a scant spoon of good quality instant coffee and juice of one lemon and 2 tspns of softened butter. Beat all ingredients until light and fluffy. If mix is too stiff, add a little milk... a drop at a time mind..... Swirl over cake top and sides and grate lemon rind over.
You can substitute lime if you like.
** for a really special frosting,- instead of the cocoa powder, melt half a block of dark cooking chocolate over warm water, mix into the butter, coffee, lemon juice and icing mix. Oh JOY!!!