This is a technique to make a boxed cake taste more like it was made in a bakery. I tried it and was very impressed.

Eggs - 1 or 2 more than the box recipe calls for. I used 1 whole and one yolk.
Milk - use this in stead of water called for on box.
Unsalted Melted Butter - Use this in place of the oil called for on the box. I actually used regular butter the first time and it was fine, but I think unsalted would be better.

Bake as directed on the box...but you might have to lower the temp a little and bake it longer; the extra eggs make it a lot wetter. 

For my most recent attempt I lowered the temp about halfway through from 350° to 325°, and ended up baking it for 45 minutes instead of the 33 listed on the box.