Line a baking sheet with foil; lightly coat foil with cooking spray. Lightly coat a metal spatula with cooking spray; set aside.
Combine sugar, syrup, butter, water, and salt in a medium saucepan. Bring to a boil over medium-high heat, stirring occasionally with a wooden spoon until ingredients are just combined. Cook, without stirring, until candy thermometer registers 300° (about 5 minutes).
Remove from heat. Stir in 2 teaspoons vanilla. Sprinkle 1/2 teaspoon baking soda over surface of mixture; carefully stir baking soda into mixture. Immediately pour sugar mixture onto prepared pan. Working quickly, spread to about 1/8-inch thickness using prepared spatula. Let stand for 2 minutes. Sprinkle chocolate evenly over toffee, and cool completely. Break into bite-sized pieces.
Note: Store toffee in an airtight container at room temperature for up to one week.