They're going to be used for baked custards!! (:
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Shuna is a pastry chef in New York City and author of the acclaimed blog Eggbeater.
You may indeed use these cups to make bain marie baked custards. The issues you may have, depending on the type of liquid you pour into them:
1. They are very light, and so - poor conductors of heat. They will float and then tip easily if you have too much water &/or displacement.
2. They can be washed & re-used but will eventually develop tiny holes.
3. Because they are aluminum I would caution against making any custards that are too acidic (caramel, molasses, citrus etc.). As well, you should eat custards within 2 days of baking/cooling, because the longer your custard sits in the cup, the more said custard will pick up a metallic taste.
Many restaurants use these to make desserts that are unfolded to order. I, for one, have unfolded thousands of molten chocolate cakes and pannacottas from them! I hope this helps - have fun :}
Thanks for that fab answer Shuna. I did actually use them for citrus custards and got another great tip from good ol' mum. She suggested to spray/brush the tins with a little oil to prevent the metallic taste when using acidic ingredients. This worked perfectly!