Place chocolate & cola in a sm saucepan and cook over low heat, stirring constantly until smooth. Remove from heat & let stand to cool. Refrigerate until cold. Beat butter til it's light & fluffy. Beat in egg yolks, one at a time. Gradually add chocolate mixture. Beat in icing sugar. Hmmm... raw egg yolks are a little dangerous these days... but I believe they can be replaced with a substitute available in cake-baking outlets.