All of the buttercream recipes I have up on the site can accommodate flavorings of various kinds. Since Opera cake is a French invention, it only seems appropriate to start with a French-style buttercream. To make a coffee-flavored buttercream, prepare a mixture of two tablespoons of instant coffee or instant espresso powder dissolved in two teaspoons of boiling water. Allow it to cool, then make a recipe of French buttercream. Beat the coffee mixture into the finished buttercream along with one teaspoon of vanilla extract. This recipe will make more than enough buttercream for the opera cake I’ll be making.