Preheat oven to 350 degrees. Grease an 8 x 8 baking pan or line with parchment paper. Set aside.
In a bowl sift the flour, cocoa powder, salt and baking powder. Whisk to thoroughly incorporate and set aside.
Heat chocolate in the microwave in 20 second intervals stirring between each until just melted. Set aside to cool briefly.
In the bowl of a stand mixer, a hand mixer works as well, combine aquafaba and sugar. Whip until mixture reaches the ribbon stage. It will be white, fluffy and when you lift the whisk it should leave a ribbon that holds it shape on the surface for a moment.
Add oil, flax egg and vanilla to melted chocolate and stir to combine.
With the mixer running, drizzle chocolte mixture into whipped aquafaba until thoroughly combined.
Switch to a spatula and add dry ingredients to aquafaba mixture and fold in until just combined.
Spread batter in an even layer in the prepared baking pan.
Bake for 25- 30 minutes or until tops are dry and a toothpick inserted comes out with only moist crumbs.
Allow to cool completely before slicing. You can cut 16 small brownies as I do or 9 larger ones.