Preheat your oven to 325 degrees. Generously grease an 8 by 8 inch square pan, and insert parchment paper with overhanging edges. This will help to remove the bars. 

In a large bowl, beat together the butter and sugar until fluffy. Beat in the vanilla extract. Add in the salt, cocoa powder, and flour, and beat for several minutes until a crumbly dough has formed. 

Spread the dough evenly in your prepared pan, and press down with the bottom of a glass for a flat, even layer. Bake for 15-18 minutes, or until the crust looks dry and just slightly darkened on the edges. Set aside. 

In a large, high-sided skillet (you can use a saucepan as well, but it may take longer), combine the butter, sweetened condensed milk, dark brown sugar, heavy cream, dark corn syrup, and vanilla extract. 

Whisking constantly, bring the mixture up to a boil over medium-high heat. Reduce the temperature to medium heat, and cook for 10-15 minutes, whisking constantly. The mixture will thicken, and become slightly darker. When you whisk the mixture, it should be thick enough that you can clearly see streaks of the bottom of your pan. 

Remove from heat, and continue stirring for another couple of minutes. Pour over your baked crust, and let the caramel set for about 30 minutes in the fridge. 

Heat the heavy cream in the microwave until boiling, and pour over the chopped chocolate in a medium bowl. Whisk until the chocolate is completely melted and the mixture is smooth. Whisk in the salt and vanilla. 

Pour over the caramel layer, and smooth out using the back of a spoon to cover the caramel layer entirely. While the mixture is still wet, sprinkle the pecans evenly over the top. 

Cover and freeze for at least an hour. Run a knife along the edges to remove the bars, and slice to your desired size. These are very dense, so I'd go small. Enjoy!