Combine first 2 ingredients and 3/4 cup water in a 2-qt. heavy saucepan. Bring
to a rolling boil over medium heat without stirring; cook, brushing down any
sugar crystals on sides of pan using a pastry brush dipped in hot water, until a
candy thermometer registers 254° (about 19 minutes). Remove from heat.
Place egg whites and salt in the bowl of a heavy-duty electric stand mixer,
and beat at high speed until stiff peaks form. With mixer running, slowly add
half of hot syrup in a thin, steady stream, beating constantly at high speed
(about 3 minutes).
Cook remaining half of syrup over medium heat until a candy thermometer
registers 270° (soft crack stage; about 4 minutes). Slowly pour hot syrup and
vanilla over egg white mixture in a thin, steady stream, beating constantly
at high speed until mixture holds its shape and begins to lose its gloss (5 to 7
minutes).
Working very quickly, drop mixture onto wax paper using 2 stainless-steel
spoons. Cool completely (about 10 minutes).