Classic Tuiles

In a medium bowl, whisk together the sugar and egg whites until well combined—you’re not beating in air and the whites shouldn’t be foamy. Add the melted butter and whisk until combined. Add the flour and vanilla (or another extract or spice, if using), and whisk to combine; the mixture should be smooth. Refrigerate the batter for at least 4 hours.

Start off by baking only one or two cookies at a time until you get a feel for the timing—they firm up quickly and can only be shaped while they're soft. 

Heat the oven to 350°F. Line a very flat, level baking sheet or cookie sheet with a nonstick baking mat (or with parchment sprayed liberally with nonstick cooking spray). Spoon a small amount of tuile batter on the baking sheet. Spread as evenly and thinly as possible into a circle the size you want (the size won’t change much during baking).

Variations:

Tuile cup or bowl: To make a cup, drape the hot cookie over an inverted shot glass, molding it as it cools. To make a bowl, start out with a bigger circle of batter and mold the baked cookie over a glass or cup with a larger base.