Melt together chocolate, butter, and coffee in the top of a double boiler over simmering water, stirring occasionally. In a small bowl whisk Kahlua into egg yolks. Then remove double boiler from heat, and gradually whisk Kahlua-egg yolk mixture into the chocolate. Set aside to cool to room temperature. In a separate large bowl beat egg whites with an electric mixer on low speed until foamy. Then turn speed to high and beat until soft peaks form. Beat sugar one tablespoon at a time into egg whites, and spoon this over chocolate mixture but do not fold it in. Beat cream in the same mixing bowl with mixer on low speed until it thickens, and then beat it on medium speed until soft peaks form. Fold chocolate mixture with egg whites into cream. Cover with plastic wrap and refrigerate until mousse is firm (at least 4 hours).