Cream the butter (with a spoon or fork) until very soft and white.
Mix in the some of the sugar add more sugar to make a firmer icing.
Mix in the drinking chocolate and mix well.
Using a knife spread the butter icing over the swiss roll half.
Once covered use a fork to lightly mark the surface of the icing to look like bark.
If you wish you can sieve a little icing sugar over your cake to make it look like snow.
Decorate with Christmas things. We used a robin and a sprig of holly (both plastic).