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.
I've made this a couple of times now and have been successful. Keep in mind the times are general. You need to watch your thermometer to make this come out correctly. It took me more like 25 minutes to get to 275. After adding the walnuts, the temp dropped then shot up very quickly. Another 5 minutes. The flavor is not terribly far off from the traditional peanut brittle recipe I have but made with less fat and calories. You could easily switch up the nuts here. Use raw nuts as the candy actually roasts them as you make the brittle. This really isn't all that hard. You just need to trust the thermometer and keep a close eye on it.