Somehow I though the cake idiom was similar to that french one: “vouloir le beurre et l’argent du beurre” (to want butter and money for butter) but it seems to differ more than I thought!

It’s about trying to exchange butter for money but thinking you could somehow end up with both, either intentionally by rigging a zero sum game to someone’s detriment, or mistakenly through a leap of flawed logic when one does not realise it’s zero sum, and thinks it can be outgamed with enough swapping around.