choc fudge icing

Hi. Could any1 tell me what makes a choc butter cream (choc, butter, icing sugar) into a choc fudge icing because they seem to have the same ingredients? Is there an addition of cream or something or is it just different proportions of the ingredients or how they are mixed?
Would anyone be so kind as to share a yummy fudge icing to fill my choc cake with?
Thank you in advance :)