This can also be made as a chocolate rum cake simply by swapping the yellow cake mix with chocolate cake mix, and the vanilla pudding for chocolate pudding. Get the driest possible cake mix, again, so that the syrup will soak in. (I haven’t tried this yet, but I plan to for birthdays! (6.25.20))
If you can’t find a super dry generic cake mix, try changing the syrup ingredients to 1/4 C each of water and rum. (I haven’t tried this yet – I found a great dry mix, for the yellow cake at least. (6.25.20))