Anyone?
I found this online, and it is copied and pasted. Personally, I would remove 1/3 cup powdered sugar, mix the rest with the cocoa, then make my royal icing from a regular recipe. You can always add the rest of the powdered sugar if you need a stiffer consistency.
Theresa 
Chocolate Royal Icing from Recipezaar
Recipe #80177
I tried to make brown royal icing to decorate bear shaped cookies. But even with good quality food color paste, I couldn't get it dark enough. This was the answer.
by Cathleen Colbert
4 cups 10 min 10 min prep
Change to: cups US Metric
1 lb powdered sugar
1/3 cup cocoa
7 tablespoons warm water
3 tablespoons powdered egg whites (meringue powder)
Mix powdered sugar, cocoa and egg whites.
Add water slowly until the icing is the consistency of thick frosting for outlining and thin with more water to fill.
I just add cocoa powder to my royal to make it chocolate, too
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