What am I, and what's the missing word?

What am I, and therefore what is the missing word?:

I was one amongst thousands
but I was the lucky one;
I came through those doors and saw you.
Eyes glazed with contentment
from courses consumed,
yet a glint in the eye showed you dared to presume
a free treat, something sweet,
something only part known
that might satisfy your curiosity.

So when you saw me your eyes lit up,
with eagerness you picked me up,
and looked at me, with untold glee,
as you tore my body in two.

I shattered into pieces, more fragile than I thought,
and as I lay there broken, you tore away some more,
then you pulled out my heart, my purpose for being.
You examined it there, I had no chance of fleeing,
you were seeing,
the message of my soul.

Then, after giving up all that once was secret,
you picked up the pieces of my broken body,
one by one,
I couldn’t run,
I endured,
as you devoured them all.

So now I’m dead.
A ghost of what I never did,
an echo of a life once lived.
Only my soul is still alive.
The message it brought only known of as lore,
but soon it will be discovered once more
in the heart of one of my brothers and sisters.
And, for the eaters, it will offer good fill in the single word here.