In a large saucepan, combine the sugar, butter, cream, 3 1/2 cups marshmallows and salt. Set over medium-high heat, and stir until boiling. Reduce heat to low and cook for about 4 to 5 minutes.

Remove from heat and add the vanilla and semi-sweet chocolate chips. Stir until melted and glossy. Set to the side and let cool slightly.

Stir in 1 cup marshmallows, 3/4 cup dark chocolate chips, and 3/4 cup pecans. Line a 9x13-inch baking dish with parchment or wax paper and then grease lightly with cooking spray. Pour the fudge mixture into the dish and spread out into an even layer with a rubber spatula. Top with the remaining 1/2 cup marshmallows, 1/4 cup dark chocolate chips, and 1/4 cup pecans.

Cover with plastic wrap and let sit at room temperature for at least 3 hours or overnight. Once first, cut into small squares and serve. Can be stored at room temperature in an airtight container for up to 4 days.