This is where the creation and use of a metaphor might be desirable.
A metaphor, according to the Merriam-Webster Website, is "a figure of speech in which a word or phrase literally denoting one kind of object or idea is used in place of another to suggest a likeness or analogy between them (as in drowning in money)."
Writers often create their own metaphors when they are searching for a figure of speech or an idiom to describe an idea they wish to express, and those already in existence either don't quite fit or sound too dull or cliché.
In this particular context, I would suggest 'cream filling' or 'creamy center,' using a metaphor of a cream-filled donut or Twinkie.
We have all bought a donut or pastry we thought was filled with delightful cream filling, and it turned out to have no filling whatsoever - this would be entirely suitable and comprehensible in this situation.
"Money Shot" is an existing option, if you don't mind a bit of sexual innuendo / metaphorical pornographic reference. Source: Google