Preheat the oven to 350°F (180°C) and line a 7x11 inch pan (about 18x28 cm) with parchment paper or aluminum foil. Set aside.
Prepare the flax eggs by mixing the ground flaxseed with the water, and let the mixture rest at room temperature for at least 5 minutes.
In a large bowl, combine the vegan butter, granulated sugar, and brown sugar. Whisk until well mixed.
Add the flax eggs and vanilla extract, and whisk again until fully incorporated.
Stir in the cocoa powder, flour, and salt until just combined.
Fold in 1 cup (180 g) of chocolate chips until evenly distributed.
Pour the batter into the prepared pan, and smooth out the top evenly. Sprinkle the remaining chocolate chips (¼ cup or 45 g) over the top.
Bake for 35-40 minutes or until a toothpick inserted in the center comes out with only a few moist crumbs. If it comes out with wet batter, the brownies need more time to bake.
Remove from the oven and let the brownies cool down in the pan for 30 minutes.
Then, using the parchment paper, carefully remove the brownies from the pan and let them cool completely on a cooling rack before slicing them into 12-16 servings.