Place the 2 cups sugar in a heavy bottomed saucepan. Stir over low heat until sugar has completely melted.
Meanwhile, in a large heavy saucepan combine the 4 cups sugar and heavy cream. Stir and heat over low.
Once the 2 cups of sugar has completely melted, pour it in a fine stream into the sugar/cream mixture while stirring vigorously. Raise heat to medium low and cook, stirring frequently, until it reaches 244 to 248 degrees F (118 to 120 degrees C) on a candy thermometer.
Remove from heat and vigorously stir in the baking soda. Stir in the butter until melted. Set aside to cool for 20 minutes. Stir in the vanilla and beat with a wooden spoon until non-glossy and it starts to set up. Stir in the nuts and spread into the prepared pan. Cool and cut into bite-size pieces.