Caramel in the bottom of the pan melds with the brownie, creating a dessert so rich and gooey that you need a spoon to serve it. Because cast iron holds heat, the brownie will still be warm when you go back for seconds.
In a medium mixing bowl, combine the 1 1/3 cups brown sugar, the flour, cocoa powder, coffee crystals, baking soda, and salt; set aside.
Heat a 10x2-inch cast-iron or other heavy oven-going skillet over medium-high heat. Add pecans; cook and stir for 2 to 3 minutes or until very lightly toasted. Add 1/4 cup butter to skillet; stir to melt, then add 1/2 cup brown sugar to skillet. Cook and stir until mixture bubbles and brown sugar begins to melt.
Remove skillet from heat and carefully add 1/2 cup cream to skillet. Return to heat; cook and stir until combined and sugar is melted. Set aside.
Add 6 tablespoons melted butter, eggs and 1/4 cup cream to cocoa mixture; stir until combined. Fold in chopped dark chocolate. Carefully spoon batter over nut mixture in skillet.
Bake in a 325°F oven for 35 minutes or until just set (place a shallow baking pan on the rack below to catch any drips). Remove from oven and cool slightly on a wire rack. Serve warm with ice cream, if you like.