Making chocolate cake out of yellow cake

My mom's birthday was last Friday, and I wasn't on the ball enough to give any forethought to making her a cake. The only cake mix I had in the pantry was a butter-recipe yellow, and I wasn't really in the mood for yellow cake. So I sifted in one-quarter cup unsweetened cocoa and then, after mixing according to the box instructions, stirred in one-half cup mini chocolate chips. I finished it off with chocolate frosting and sprinkles (because, according to Faith, all cakes have to have frosting and sprinkles), but I think it would have been excellent with just a dusting of powdered sugar; the frosting took it over the edge into way too sweet. The flavor of the cake was very good, though -- less overpowering than a straight chocolate cake mix would have been. If I make it again, I'll leave off the frosting and probably dust the Bundt pan with cocoa instead of flour.