Line a large baking sheet with parchment paper. Alternatively, you can just set the parchment paper out on your countertop.
In a large saucepan, combine the sugar, water, butter and corn syrup and bring to a boil. Cook over medium high heat, stirring occasionally, until the caramel is light brown and reaches 300° F on a candy thermometer, about 10 minutes. Note that around 250° F the mixture will start to thicken.
Remove from the heat and carefully stir in the baking soda.
Stir in the nuts, then immediately scrape the brittle onto parchment paper. Spread the brittle into a thin, even layer. Work fast - it will set quickly. Sprinkle with salt as soon as you can so that it will stick.
After the brittle cools for at least 30 minutes, you can break it into large chunks.
Stored in an airtight container at room temperature for up to 1 month.