It is a dessert, which can be made by sims with sufficient cooking skill points. It is only available with expansion packs OFB or after.
I'll get back to you soon if as to if it works, I'm trying it
right now! (I hear it only improves chances of twins)
On a side note, I once had a couple who always had twins, no cheats or anything. It was purely genetic, even though the mother was not a twin (not so sure about the father, he was a townie).
I believe it improves the chance of twin by quite a bit each time your sim eats a slice, so a whole cake will almost guarantee twins, whenever I want twins I make three or four cakes and it has never failed.
or you can just do "boolprop testingcheatsenabled true" in the cheat box, (ctrl + shift + c) and then click "shift", click your pregnant woman, then click "spawn" then click "tombstone of L and D" "more" and the button that says "forcetwins" and after that i usually do "speed up my pregnancy" to make it faster