Ask a Vegan Baker

Hi,
I have a recipe for chocolate frosting that tastes very good, but for some reason, I am left with white streaks and a shaggy texture when I've finished incorporating and beating all ingredients. Here is the recipe:

3 1/2 cups confectioners sugar
1/2 cup cocoa powder
1/2 cup coconut oil, solid
2 tsps. vanilla
1/3 cup soy milk

First, I sift sugar and coco powder together to strain any lumps. In the bowl of a standing mixer, I beat coconut oil for about 30 seconds then add vanilla. In heaping tablespoons, I add sugar and cocoa powder mixture and drizzle in the soy milk. When all is incorporated, I beat for 5-6 minutes on medium high. Frosting has white streaks throughout and is shaggy as opposed to fluffy. Any thoughts or advice would be most appreciated. Thanks!