In a small saucepan, melt the margarine over low heat until just melted. Remove it from the heat immediately. In a medium mixing bowl sift together powdered sugar and cocoa powder. Pour margarine in another medium mixing bowl. Allow to cool a bit. Add cream cheese to margarine and mix with electric mixer at medium speed until combined. Gradually add the sugar/cocoa mixture. Add coffee liquor and vanilla and beat slowly. This will thin the frosting. Adjust to taste. If the frosting is too thin add more sugar and cocoa. (It will thicken a bit as it chills though.) Chill frosting for at least a couple of hours. Frost thoroughly cooled cake.
Note: This frosting is excellent on a chocolate cake, but can be used on any flavor of cake with which you think it will go. And to punch up the flavor of that chocolate cake, try substituting 1/2 of the liquid in the recipe with very strong coffee. This will give the chocolate a richer flavor.