The Mogen Avraham (O.Ch.430) writes that the Maharshal used to Darshen that "everybody should bake a Challah (before Pesach) using the Pesachdike flour, so if there was any Chometz in the flour, we will assume that the Chometz was baked in the Challah and eaten.
It is also a good thing to give this Challah to an Oni (poor man)
The Noheg Katzon Yosef quotes the Rosh
“The Minhag on Erev Shabbos Hagodol is to bake
חלת עני from the whole Kehilla and distribute it to the Aniyim. From the time this Minhag was neglected, a curse was sent on the grain.”
Four corner Challah
The Kol Bo (52) writes that there is a minhag for women to bake on Shavuos a long bread with four corners lezecher the Shtei Halechem
(looks like 2 breads) or because Mazel Sivan is Teomim (twins)