The shell is the mould
A madeleine is a small butter sponge baked in a tray of shell-shaped hollows. The ridged shell that makes it recognisable is not a technique and not an achievement: it is the shape of the tin, and any batter poured into that tin comes out with it.
What French bakers actually judge a madeleine on is the hump, or la bosse: the raised dome that forms on the flat side while it bakes. A madeleine without a hump is held to have failed, however good the shell looks.
That inversion is the useful content of this page. The visible signature is free; the actual standard is the part most people photograph face-down.
What the hump is credited to
The hump is universally attributed to thermal shock: a cold, well-rested batter meets a hot oven, steam forms fast in the centre, and the middle rises before the outside has set enough to hold it down. On that account the batter has to be genuinely cold and genuinely rested, and the oven genuinely hot, and that yields a practical instruction rather than a vague one.
It is worth being clear about the standing of that explanation. It is the standard professional account and it is given the same way by everyone; nothing consulted for this page measured a temperature differential or a rise. So it is recorded here as the accepted explanation, not as a demonstrated one. The instruction it produces, to rest the batter cold and bake it hot, is what to act on either way.
The mould matters for the same reason it matters to a canelé: heat getting into the batter fast and unevenly is the whole mechanism, so the tin is part of the specification rather than an accessory.
The 1755 story, and what it is worth
The madeleine is a cake of Commercy and Liverdun, two communes in Lorraine. The usual story has a servant named Madeleine Paulmier improvising it at Commercy in 1755 for Stanislas Leszczyński, the deposed Polish king then Duke of Lorraine, who was himself a keen baker.
It is told as a legend, and it should be read as one. Whether Madeleine Paulmier existed is not established, and which of the two towns has the older claim is not settled either. Both towns still claim it, which is generally a sign that nobody can prove it.
Adding raspberries, as this page’s original title did, changes nothing structural but does raise one thing worth knowing: raspberry is recorded in the fruit register as medium acid with its pectin not established, and it carries a good deal of water. Fresh berries pressed into a sponge batter release that water where the crumb is trying to set. The rest of the technique pages are collected in the technique section.
Questions about madeleines
What is the hump on a madeleine?
La bosse, the raised dome on the flat side. It is what French bakers judge a madeleine on; one without a hump is considered to have failed however good the shell looks.
How do you get a hump on a madeleine?
The standard account is thermal shock: a cold, well-rested batter into a hot oven, so steam raises the centre before the outside sets. It is the accepted professional explanation rather than a measured one, but the instruction it yields is clear enough.
Where does the shell shape come from?
The mould. It is a tray of shell-shaped hollows, so the shape requires nothing of the baker, which is why it is not what the cake is judged on.
Did Madeleine Paulmier invent the madeleine in 1755?
Not established. The story is told consistently, with a servant of that name improvising it at Commercy for Stanislas Leszczyński, and documented nowhere. Commercy and Liverdun both still claim the cake.