There are dairy free and soy free margarines, if you can find them. All the recipes I looked at asked for this, but I could never find it, then again I am in a smaller city in Canada. Or you can use shortening, and find one soy free, that's what I did. Keep in mind that he's only turning one, if the cake doesn't have icing he won't notice. I worked real hard making him a cake and he didn't even touch it ahahahaha!!! Thankfully we had a back-up cake for the guests, because the rice flour cake with shortening icing wasn't a HUGE hit with the adults... lol
Here is the ingredients I used::
4 cups powdered sugar (1 pound)
2 Tbsp water, or rice milk
1 cup shortening
1 tsp clear vanilla extract
1/2 tsp butter flavoring (optional)
paste food coloring (optional)