Pre-heat the oven to 350 ºF (175 ºC) and line a 8 x 8 inch square baking pan with baking/greaseproof paper.
In a heat-proof bowl above simmering water, melt the chocolate and coconut oil. Set aside to cool until warm.
In a stand mixer, or using a hand mixer with double beater attachments, whisk the aquafaba, sugar and cream of tartar for at least 10 minutes, until stiff peaks form.
Fold the melted chocolate into the whipped aquafaba until you get an even mixture with no streaks of chocolate. Be gentle here - you don't want to lose too much air!
Sift together the flour, cocoa powder, baking powder, baking soda and salt, and add to the chocolate mixture. Gently fold everything together until you get a smooth mixture with no flour clumps.
Transfer to the lined baking pan, and smooth out the top with a spatula or spoon.
Bake in the pre-heated oven at 350 ºF (175 ºC) for 18 - 22 minutes, or until an inserted toothpick still comes out slightly "dirty" (with plenty of moist crumbs).
Allow to cool slightly, and serve - preferably with some vegan ice cream and salted caramel sauce. (See Note 2.)
