Preheat oven to 350°F and line 8x4-inch baking tin or loaf pan with parchment paper.
Mix together ground flaxseeds with water in small bowl, and set aside.
Place nuts and dates in bowl of food processor fitted with blade attachment and pulse to roughly combine. Add cacao, baking soda, vanilla extract, salt, and flaxseed mixture and process thoroughly until ingredients form a homogenous batter. Stir in chopped chocolate.
Scrape batter into prepared tin and bake 10 minutes in oven. Cool completely before cutting into bite-size pieces.