You can use this trick to make a box cake mix taste like cupcakes or cake are made from scratch.
*In case your wondering if this will work with the box cake mix you want to use: I've been making cupcakes and cakes this way for over 10 years now, I'm pretty sure box cake mixes have changed in size slightly over the years, but this still works.
Basically, you want to just use the egg as directed on the box — butter in place of oil and just a little less water than the mix says to use.