How do you record food that is not in the database and you have no idea what's in it? In this case a piece of birthday cake. I never eat such things and could not say what it was.
I would recommend searching for something from NCCDB that is your closest estimate of the food you ate away from home.
Birthday cake, as an example, I would use white cake and then also add Frosting, Chocolate, Ready to Spread or Frosting, White or Flavored, Ready to Spread (depending on the flavor) and estimate my serving sizes.
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