Line a baking sheet with parchment paper. In a medium saucepan, combine the sugar, water, corn syrup and butter and bring to a simmer. Cook over moderate heat, swirling the pan a few times, until a light caramel forms, about 4 minutes. Stir in the baking soda and salt. Remove the pan from the heat and stir in the walnuts. Quickly spread the toffee on the prepared baking sheet as thinly as possible. Let cool completely. Break into pieces and serve.