Preheat the oven to 325°F, grease an 8x8-inch baking dish and line it with parchment paper.
In a small bowl, stir together the flaxseed and water and set aside for 5 minutes to thicken.
In a medium bowl, combine the flour, sugars, cocoa powder, baking powder, salt, and chocolate chips.
In a large bowl, whisk together the oil, vanilla, and the flaxseed mixture. Pour the dry ingredients into the bowl of wet ingredients and use a spatula to fold the mixture together, until moistened. It will be extremely dry and crumbly but will come together in the next step where it’ll be pressed into the pan. This mixture is very different from traditional brownies where a wet batter is pourable.
Transfer the crumbly mixture to the prepared pan and use your fingertips to press it into an even layer. Sprinkle the reserved chocolate chips on top and gently press into the dough. Sprinkle with flaky sea salt, if using. Bake for 25 minutes, or until a toothpick inserted in the center comes out with just a few crumbs. Cool on a wire rack for at least 45 minutes before slicing and serving.