In a small saucepan (preferably with a heavy bottom), pour in 2 cups of sugar and cook over low heat, stirring often. Cook until the sugar is completely melted, which takes about 20 minutes.
While the sugar is cooking down, in a large saucepan (or Dutch oven), combine the remaining 4 cups of sugar and heavy cream. Simmer over low heat until the sugar in the small saucepan has melted. Add the melted sugar to the large saucepan while continuously stirring.
Bring the heat up to medium and cook the mixture until it reaches 244 degrees F or the softball stage.
Remove from heat and stir in baking soda until completely combined.
Add butter and vanilla. Stir until the butter has melted, and the mixture is smooth.
Let the mixture rest for 20 minutes.
After the mixture has rested, stir in nuts and mix until the sheen has disappeared. Transfer the mixture to the prepared baking dish.