Well, since we know they combined
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×eggs, sugar, salt, baking soda, and cream of tartar
I'd have to assume that the other chemicals were
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×milk, vanilla extract, and flour
and request that, if you run the experiment yourself, you send the resulting solids along to me.
(Note: sadly, when doing some basic research on these compounds:
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×the first Google result for KHC4H4O6 was
this page, which had basically the same puzzle; fortunately, I knew what was going on enough so that the brief excerpt from Google didn't actually spoil anything for me.