Stir sugar, corn syrup and water over low heat until sugar is dissolved. Cook, without stirring to 260ºF on candy thermometer or until small amount of mixture dropped into very cold water forms a hard ball that holds its shape but is pliable.
Beat egg whites in a stand mixer on high speed until stiff peaks form. Continue beating while pouring hot syrup in a thin stream into egg whites, beating constantly on medium speed. Add vanilla. Beat until mixture holds its shape and becomes slightly dull. (If mixture becomes too stiff for your mixer, you'll need to beat it by hand.) Gently stir in nuts if using.
Use two spoons to push dollops of candy onto parchment paper or a Silpat. Let stand at room temperature until candies feel firm. Store in airtight container or wrap in plastic wrap to give as gifts.
Notes
*Can sub traditional vanilla extract**Use 1 tablespoon less water on humid days and let dry longer on parchment paperAdapted from Betty Crocker