Cook sugar, corn syrup, and water in a 2-quart pot over low heat, stirring constantly, until sugar is dissolved. (On humid days, use 1 tablespoon less water.) Cook, without stirring, to 260 degrees on candy thermometer (or hard ball stage).
Beat egg whites in 1-1/2-quart bowl until stiff peaks form. Continue to beat while pouring hot syrup in a thin stream into egg whites. Add vanilla; beat until mixture holds its shape and becomes slightly dull. (Mixture may become too stiff for electric mixer). Fold in nuts (if you choose).
Drop from buttered spoon onto waxed paper. Let stand at room temperature, turning candy over once, until outside of candy is firm–at least 12 hours. Store in airtight container.
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