Depending on how much egg you are using in your mixture, and whether you bake or fry your French toast, sometimes the custard mixture puffs up (smaller version of a souffle) while baking.
When you take it off the heat there will be, as Phil says, some deflation.
I think you are going to get some deflation. What bread are you using? It needs to be spongy but sturdy, 3/4 - 1" thick. Is it cooked through? if it is too soggy it will sag. Make sure the pan is hot before you put the toast on it.
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When you take it off the heat there will be, as Phil says, some deflation.