Seven minute frosting has egg white, sugar, corn syrup, water , flavour.
Coconut milk from a tin has no milk, no lactose. That can be used in place of milk/cream in "creamed" icings and custard fillings.
Make sure you buy certified vegan or certified dairy free margarine.
Many chocolate cake recipes use coffee NOT milk, and oil. These recipes use cocoa, or unsweetened chocolate, to avoid traces of milk in chocolate.
Best to do a google...