Cook first 4 ingredients in a heavy 2-quart saucepan over low heat until sugar dissolves and a candy thermometer registers 248 (about 15 minutes).
Remove syrup mixture from heat.
Beat egg whites at high speed with an electric mixer until stiff peaks form.
Pour half of hot syrup in a thin stream over egg whites, beating constantly at high speed, about 5 minutes.
Cook remaining half of syrup over medium heat, stirring occasionally, until a candy thermometer registers 272 (about 4 to 5 minutes). Slowly pour hot syrup and vanilla extract over egg white mixture, beating constantly at high speed until mixture holds its shape (about 6 to 8 minutes). Stir in 1 cup chopped pecans.
Drop mixture quickly by rounded teaspoonfuls onto lightly greased wax paper.
