In a small saucepan (with a heavy bottom if you have it!), pour in 2 cups of sugar and cook over low heat while stirring often. Cook until sugar is completely melted, about 20 minutes.
As sugar cooks down, add the 4 cups of remaining sugar and heavy cream to a large saucepan (or dutch oven, you want a big pot here!). Simmer over low heat until the sugar in your small saucepan has melted. Add melted sugar to a large saucepan with continuously stirring.
Bring the heat up to medium and cook mixture until 244 degrees F or softball stage.
Remove from heat and stir in baking soda until completely combined.
Add butter and vanilla. Stir until butter has melted and the mixture is smooth.
Let rest for 20 minutes.
After the mixture has rested, stir in nuts and mix until the sheen has gone. Transfer to prepared baking dish.
Let cool 1-2 hours or overnight. Cut into small squares to serve.