Spray the baking pan with oil. Alternatively, line an 8x8-inch (20×20 cm) pan with parchment paper. Preheat the oven to 350°F (176°C).
In a medium bowl, mix the all-purpose flour and cacao powder, and set aside. Add the aquafaba (egg replacement) and monk fruit sweetener or sugar to a blender and blend on high for about 2 minutes.
Or, add them to a large mixing bowl. Use an electric handheld mixer or a stand mixer at high speed and beat the mixture for 2 to 3 minutes until thickened and shiny.
Next, pour in the vanilla and salt. Combine with a spoon or silicone until well incorporated. Prepare the double boiler. Use a heatproof bowl that can fit well into a saucepan.
Fill the pan up with a couple of inches of water - not enough that the bottom of the bowl would touch the water. Following that, heat the water on the stove and keep it at a simmer.
Add the chocolate chunks and vegan butter to the bowl on top of the pan. Next, let the chocolate mixture melt and whisk until smooth and completely melted. Once melted, remove the bowl and turn off the stove.
Add the warm chocolate-butter mixture to the aquafaba-sugar mixture and fold it together with your silicone spatula or spoon.
Mix the wet and dry ingredients. Add the flour-cacao mixture to the chocolate mixture and stir until well incorporated (mix until the flour traces are gone).
After that, fold in the chocolate chips and nuts (if applicable). Pour the brownie batter into the pan. Spread out the surface with the spoon.
Bake the brownies for 30-37 minutes (times will vary depending on your oven), insert a fork (or toothpick) in the middle and it should come out with some moist crumbs. Note: Check them a few minutes before you believe the brownies will be done. A little gooey batter is fine. If the brownie is wet, the brownies most likely need an additional 5-6 minutes.
Let the brownies cool completely for 30 to 50 minutes. Slice and enjoy!