Preheat your oven to 350°F (approx. 177°C). Grease an 8-x-8- inch pan with cooking spray or oil. Measure a sheet of parchment paper approximately 16 1/2 x 6 inches in size and lay it across the center of the greased pan. Press down so the paper sticks to the oil. In the end, you should have about 3 inches (8 cm) of paper overhang on two opposite ends of the pan; these will serve as handles to later lift the brownies out of the pan. Set the pan aside.
In a small bowl, combine the flaxseed and water and set aside.
In a mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, and salt.
Add the oil, milk, vanilla extract, and flaxseed mixture to the bowl and stir to combine. The batter will be very thick.
Gently fold in 2/3 c. (124 g) of the chocolate chips.
Transfer the brownie batter to the prepared pan and gently spread so that it reaches all sides and corners. Pat the surface lightly with your fingertips to help smooth it over.
Sprinkle the remaining 1/4 c. (47 g) of chocolate chips evenly over the surface.
Bake the brownies in your preheated oven for 23–25 minutes, or until the surface is set and the edges have just started to puff up and pull away from the pan.
Place the pan on a cooling rack to cool. Note: The brownies will likely sink in the center as they cool. This is perfectly normal and is a sign that your brownies will be extra fudgy.
Once the brownies are cool, place the pan in the fridge for at least an hour or until you’re ready for them. Then, while still cold, slide a thin knife along the two sides of the pan not covered in parchment paper. Using the parchment handles, lift the brownies out of the pan and set them on a cutting board. Cut into 16 squares.