I found it here on Allrecipes. I'll put my adjustments in italics.
1/3 cup unsweetened cocoa powder (I used two heaping spoon fulls because i didn't want it to overpower the coffee flavor. Just play it by ear. You may want to use more)
1/3 cup strong, hot, brewed coffee (I brewed an entire mug of coffee but used two or three times the grounds as it would normally require. I watered the leftovers down and drank it while the cake was baking!)
3 cups confectioners' sugar (I used 4 cups. It was kind of runny.)
1/3 cup melted butter (use whatever alternative you'd like. You might be able to skip this actually since it was kind of runny.)
1/8 teaspoon salt
1 teaspoon vanilla extract
1/4 cup chopped walnuts