Line a jelly-roll pan with parchment paper, and coat paper lightly with cooking spray.
Combine sugar, light-colored corn syrup, 1/2 cup water, and butter in a heavy saucepan. Cook over medium heat, stirring until sugar dissolves. Cook 20 minutes or until a candy thermometer registers 275°. Stir in walnuts; cook 2 minutes or until candy thermometer registers 295°, stirring constantly. Remove from heat; stir in baking soda and vanilla (mixture will bubble). Quickly pour mixture onto prepared pan; spread to 1/4-inch thickness using a wooden spoon coated with cooking spray. Cool completely; using a wooden spoon, break brittle into bite-sized pieces.