It almost sounds like you didn’t mix the chocolate thoroughly enough into the buttercream before frosting the cake, could that be it? Or could the buttercream have been too cool when you beat in the melted chocolate, so that lumps formed?
It’s probably not a good idea to increase the amount of chocolate in the buttercream, because too much cocoa butter can make a frosting too stiff. And mousseline is already the firmest of Rose’s buttercreams. Cocoa powder has less cocoa butter than bar chocolate, and I know many who, as Rozanne notes, love Bill’s chocolate buttercream.
The only other thing I can think of is perhaps it was the combination of too much cocoa butter in the buttercream, on a too cool cake, so that the cocoa butter firmed up and was difficult to smooth out.
Lastly, I would consider adding the liqueur in the recipe, as this creates the smoothest texture. Cognac, Grand Marnier, Frangelico and Kahlua all are wonderful with chocolate.