1/2 cup unsweetened cocoa powder (sifted if there are clumps)
2 1/2 cups powdered sugar, sifted
3 tablespoons soy milk
1 1/2 teaspoons vanilla extract
Cream together margarine and shortening in the bowl of a stand mixer fitted with the paddle attachment. Add cocoa powder and incorporate well.
Add powdered sugar in 1/2 cup batches and beat well, adding a little splash of soy milk after each batch. When all ingredients have been incorporated, add vanilla and beat until light and fluffy, about 3 minutes.