OVEN: 250 Cover 2 baking sheets with parchment or foil.
Beat egg whites with salt til soft peaks form. Add sugar very slowly, beating well between additions, til egg whites are stiff but not dry.
Fold in lemon zest and vanilla.
Drop by teaspoonfuls onto the baking sheets or pipe small kisses with a pastry bag fitted with a 1/2-inch star tip.
Bake til dry, but not brown, 40 to 45 minutes. Cool 2 to 3 minutes before removing from baking sheets.
(Can be stored up to 2 weeks in an airtight container.)