Add egg whites and vanilla extract, whisking until fully blended. Dough should be thick but moist (like a very thick cake batter)
Place spoonfuls of dough on prepared cookie sheet, leaving space for cookies to spread
Bake for 12-14 minutes, until tops are glossy and cracked.
Let cool for 5 minutes on sheet, then transfer to cooling rack (I have found this is easiest if I leave the cookies on the parchment, then remove the paper when the cookies are completely cool)