Coat a baking sheet with cooking spray. Cook hazelnuts, sugar, and corn syrup in a saucepan over medium-high heat, stirring constantly, until sugar dissolves.
Clip a candy thermometer to side of pan, and continue to cook, stirring occasionally, until mixture is golden brown (290 degrees). Stir in salt and baking soda (mixture will foam), and immediately pour on prepared sheet (do not spread). Let stand until hard, about 2 hours. Break into pieces. Using kitchen tweezers, decorate brittle with gold leaf.