Line a heavy large baking sheet with parchment paper. Stir the sugar, water, light corn syrup, and dark corn syrup in a heavy large saucepan over medium heat until the sugar dissolves. Increase the heat to high and boil without stirring until a candy thermometer registers 260 degrees F, about 20 minutes. Reduce the heat to medium-low. Mix in the hazelnuts, butter, and salt (the mixture will be thick and nutty), and cook until the thermometer registers 295 degrees F, stirring constantly, about 15 minutes. Quickly stir in the baking soda. Immediately pour the caramel onto the prepared baking sheet, spreading it as thinly as possible. Let stand until hard.
Break the brittle into pieces and store in an airtight container at room temperature. If desired, serve over your favorite ice cream.