The odd ingredient in a cake is, to me, different if it is not something likely to be objected to, like the apple sauce vs. butter, salt in caramel, even a tomato although I'm not seeing how that would be good, or something like that. Rarely is one asked the recipe before taking a bite, but if you were asked, I think you need to reveal all that went into it.
I think it was clear to all involved that you thought it was eggs and wouldn't have eaten it if you thought it was brains. I'd probably go down to dinner, go to the kitchen and make something I wanted to eat. Your mother will no doubt ask why you are not eating the dinner she cooked and the answer is plain; you don't trust her to prepare food and tell you what it is. It seems such a clear violation, the solution is to cook for yourself in the future.