Preheat the oven to 350 degrees. Line a 9x13 cake pan with parchment paper on the bottom and sides.
In a very large bowl, whisk together the 2 brownie mixes, instant clearjel, and dark dutch cocoa powder.
In a separate bowl, whisk the eggs until very smooth, then mix in the vegetable oil, water, and vanilla extract.
Pour the wet ingredients over the dry ingredients, and fold the two together using large strokes. Continue to fold until there are no streaks of dry ingredients (avoid overmixing).
Fold in 1⁄2 of the chopped chocolate bar and 1⁄2 of the chopped pecans (if using), then spread the brownie batter into the prepared baking pan. Sprinkle the remaining chocolate chunks and chopped pecans over the top of the brownies.
Bake in the preheated oven at 350 degrees for 20 minutes, then reduce the temperature to 325 degrees. Bake for an additional 35-50 minutes or until a toothpick comes out clean throughout. (Baking time will vary depending on the oven and pan used, so use a toothpick or pairing knife to test the center of the brownies for doneness. Allow to cool completely before serving. These brownies have the best texture the next day or after cooling for at least 3 hours. Enjoy!