Make a scalloped edging around the Pithiviers as follows: set an upturned bowl slightly smaller than the Pithiviers over it, and use it as a guide in cutting 2-inch-wide (5 cm) scallops all around the circumference; decorate their edges all around by pressing upright lines against them with the back of a knife.
Immediately set the Pithiviers in the preheated oven and bake for about 20 minutes, until it has puffed and is beginning to brown nicely -it should rise dramatically, to a height of at least 2 inches (5 cm).
The Pithiviers should be done when the sides feel quite firm; to be sure, turn oven off and leave the Pithiviers in for another 10 to 15 minutes.