In a double boiler, over hot water (not boiling), melt chocolate pieces and margarine. Stir in sugar and soymilk, and remove from heat. Beat mixture with a spoon until smooth.
Variation: Instead of soymilk, you can use hazelnut or oat milk.

2 cups non-dairy chocolate chips
1/4 cup margarine
3 cups powdered sugar
1/2 cup soy milk