I would like to bake an [Opéra cake](http://en.wikipedia.org/wiki/Opera_cake) for someone who doesn't really appreciate the taste of coffee. Since coffee and chocolate are the two main elements, I am wondering: what flavour could I substitute for coffee in my coffee buttercream? I suppose the qualities I'm looking for are a bit of bitterness, and it should go well with chocolate and orange liquor?

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PS: I just noticed that the English wikipedia describes the sponge cake as “soaked with coffee”, but in the way I usually do it it's soaked with [Cointreau](http://en.wikipedia.org/wiki/Cointreau) so I don't need any replacement there.