These unusual cookies get a one-two punch of fl avor from sesame seeds, which are known as benne [BEHN-ee] in the South. Rolled in toasted seeds to give them a coating with delicate crunch, these crescents include a surprise ingredient that gets stirred into the dough. Toasted sesame oil, usually relegated to Asian cooking, rockets the sesame flavor of these cookies straight to the moon.
Light-colored sesame oil is usually found in grocery stores. Darker-colored oil, labeled “toasted sesame oil,” has a more pronounced sesame flavor, which is why we prefer it in this cookie.
Toast sesame seeds in a dry skillet over medium heat, stirring constantly, until beginning to brown, about 5 minutes; transfer to a bowl.
Whisk together flour and salt in a bowl; set aside.
Cream butter and powdered sugar with a mixer on medium speed until smooth, about 5 minutes.
Add egg, sesame oil, and vanilla to creamed mixture; beat on low speed until well combined. Add half of the flour mixture; beat on low until ingredients are combined. Add remaining flour, beating just until flour is incorporated into dough. Wrap dough in plastic wrap; chill at least 1 hour.
Preheat oven to 350°. Line baking sheets with parchment paper.
Lightly beat egg white and water in a small bowl.
Using your hands, roll dough into 1-inch balls; shape each dough ball into a crescent.
Dip each crescent into egg white mixture, letting excess drip off, then dip crescents into seeds. Transfer cookies to prepared baking sheets, spacing 2 inches apart.
Bake cookies, one sheet at a time, until firm to the touch and lightly brown on the bottoms, 13–15 minutes. Let cookies cool on the cookie sheets 5 minutes, then transfer to a rack to cool completely.
Make-Ahead Directions: Prepare and shape dough into cookies; coat each with sesame seeds. Freeze molded cookies on a baking sheet, then transfer them to a resealable plastic bag and store in the freezer up to 1 month. Thaw cookies on a parchment-lined baking sheet before baking as directed.