Preheat the oven to 350 degrees and prepare a 12 x 18 x 1-inch baking sheet with cooking spray and parchment paper on top.

Combine the flour, cocoa powder, sugar and salt in a medium bowl. Set aside.

Whisk the eggs, oil, water and vanilla in a large bowl.

Add the flour mixture into the bowl with the wet ingredients and stir until just combined. Add the walnuts and mix until just incorporated. 

Spread the batter evenly into the prepared baking sheet and bake for 30-35 minutes until a cake tester comes out with some crumbs attached. The brownie is fudgy, so the tester should not come out clean.

Cool brownies to room temperature.

Melt the chocolate: Prepare a double boiler by filling a small pot with 2 inches of water. Put the pot over medium-low heat and bring the water to a gentle simmer.  Place a small bowl on top of the pot. Place the chocolate chips and oil into the bowl. Whisk the chocolate constantly until it is melted, smooth and has a glossy texture.

Garnish the brownies: working quickly, invert the brownies onto a large cutting board, or over a wire rack a baking tray. Pour half of the melted chocolate over the brownies and use a spatula to spread the chocolate evenly over the top surface of the brownies. Transfer the brownies to a refrigerator for 10 minutes until the melted chocolate has hardened. Place another tray or cutting board on top of the brownies and invert the brownies to reveal the bottom side of the brownies on top. Pour the remaining chocolate on top of the brownies and spread the chocolate evenly. Transfer the brownies into the refrigerator for 10 minutes until the chocolate has set.

Cut the brownies: dip a long sharp knife into warm water and then dry the knife with a towel. Cut the brownies into even 1 inch squares. 

Serve at room temperature.