Preheat oven to 350 degrees. Oil an 8-inch square pan and then line with parchment paper so the ends extend up over the top of the pan. Lightly oil the parchment paper. In a large bowl, combine the cocoa powder, potato starch, tapioca flour, sugar and salt. Whisk in the eggs, oil, water and vanilla. Stir until well combined and uniform in texture. Pour the batter into the prepared pan and bake until a toothpick inserted in the center of the brownies comes out almost clean, about 40 to 50 min. Keep the brownies in the pan and refrigerate for 1 hour. Using the ends of the parchment paper, lift the brownies out of the pan and cut into squares.
With your favorite ice cream!