01 -
Whisk the water, vanilla extract, and oil together. If using flax seeds, whisk them in as well. Set the mixture aside.
02 -
Preheat the oven to 330°F (165°C). Grease a 9x13-inch pan or an 8x8-inch pan for thicker brownies, or line the pan with parchment paper.
03 -
In a large mixing bowl, stir together the cocoa powder, Dutch cocoa powder, flour, salt, baking powder, brown sugar, white sugar, and cornstarch.
04 -
Gradually add the liquid ingredients into the bowl with the dry ingredients and mix to form a smooth brownie batter.
05 -
Smooth the batter evenly into the prepared pan.
06 -
Bake for 16 minutes in a 9x13-inch pan, or 24 minutes in an 8x8-inch pan. The brownies should appear slightly underdone when removed from the oven.
07 -
Let the brownies cool completely, then refrigerate uncovered or very loosely covered. They will firm up after a few hours and are easier to slice the next day.
08 -
Keep leftover brownies refrigerated in an airtight container for up to 4 days or freeze them for up to three months.