Put it in a lightly floured bowl, make an X in its surface, cover it with a cloth, and put it in a warm place to rise. Sift the flour into a bowl, then add the salt, the sugar, the vanilla or lemon zest, and mix. Break the butter, which should be soft, into bits, and incorporate it. Then incorporate the biga, kneading the dough energetically and throwing it down on your work surface, until bubbles appear in the dough and it becomes shiny. When the time is up return the dough to your work surface and knead it for a few more minutes, then return it to the bowl and put it in a cool place overnight.
Pastry chefs prepare the dough in the evening to have it ready the next morning.
To make brioches you'll need the dough, and egg, and a little butter and flour. You'll also need brioche molds, which are small cups with scalloped edges. Ms. Gosetti suggests you make rings of paper if you don't have them, but a muffin tin should also work. Assuming you have the molds or the muffin tin, melt a little butter and brush the insides of the tins, then flour them lightly, upend the tins, and shake out the excess. Shape the larger pieces into balls by rolling them between your palms, and put them in the molds, which they should only fill about half way.