In a small bowl whisk together the water (3 tablespoon) and flax meal. Let it sit for about 10 minutes till it becomes frothy. This mixture is called flaxseed egg. Coat the chopped walnuts with little flour to avoid sinking to the bottom. In a medium bowl whisk together flour and baking soda. Keep aside.
Preheat oven to 180 °C. Line & grease an 8X8 inch baking dish. Allow overhang on all sides for easy lifting of brownies after baking.
In another medium bowl add the cocoa powder, chocolate, coffee and salt. Add boiling water and mix in the ingredients into a paste using a spoon. Make sure that chocolate pieces are melted. Add jaggery powder, butter, vanilla extract and flax meal egg into the chocolate mixture until smooth.
Stir in the walnuts if you're using them along with grated cucumber. Using a wooden spoon, mix in the flour until well combined. The mixture becomes extremely thick and you may need to use your hands to mix. Don't mix the dough with an electric mixer at this point. It will overwork the dough and gluten will form, in turn affecting the texture of the brownies.
Transfer the batter to the baking dish, using a silicone spatula press the thick dough into place. Bake for 30 minutes. Cool on a wire rack for an hour. Transfer the brownies to the wire rack by lifting the overhang. Allow them to cool completely before slicing into squares.
Serve with powdered sugar or ice-cream or sauce or eat them off the rack. Enjoy!