Lite Licks soy icecream - the chocolate one - is the only soy icecream that I liked. I tried a few, and some even my sow wasn't overly impressed with!!
You can make fruit crumbles/sponges etc with margarine in place of butter but they are not especially decadent.
Whittakers dark choc is dairy free, so you could make something with that (or just eat that!!).
Perhaps a really rich chocolate cake with marg in place of butter and whittakers dark choc? Actually, pams dark choc chips are dairy free too.
If it makes you feel any better, DD went from 20 hours of desperate screaming in response to me eating a sandwich with dairy in the margarine when she was 5.5 months old, to me drinking a cup of tea with milk (I had a brain explosion and forgot I was dairy free - whoops!!) and not seeing any response from her when she was 7.5 months old. These days, she can even eat the dairy herself, although in limited quantities. So I know it's hard, but it's not forever