Fluffy and delicious.
L'école française sais that the colder the dough is kept, the more likely it is that the madeleine will form the classic bump on the back.
The ideal would be to prepare the dough of the madeleines the night before and leave the dough to rest in the fridge all night. I have experimented that dividing the dough into the madeleines tray shells and putting it in the freezer for about 5-10 minutes, has the same effect.