Preheat your oven to 170°C. Line an 8x8" or 7x11"' baking pan with parchment paper and set it aside. Separate out 100g of the dark chocolate and cut it into chunks, set it aside.
Add the remaining 120g of chocolate to a heat-proof bowl and melt it over a double boiler. Remove from the heat, whisk in the rapeseed oil and set aside.
In a small bowl or cup, mix together the soy milk and apple cider vinegar to make a vegan buttermilk mixture and set aside.
Add the aquafaba to a separate large bowl and beat using an electric whisk for 5 minutes until the mixture is light and fluffy.
Mash 2 of the bananas with a fork in a large bowl. Add the coffee, coconut sugar, and vanilla extract and whisk the mixture to combine. Add the vegan buttermilk mixture and whisk again until mixed. Then add the melted chocolate mixture and whisk for another 30 seconds to combine. Carefully fold the whipped aquafaba into the bowl with the wet ingredients using a spatula.
In a separate large bowl, mix together the flour, cacao powder, baking soda, and salt. Gently fold the dry ingredients into the bowl with the wet ingredients in 2-3 increments, do not over-mix. Lastly, fold in the reserved chocolate chunks (you can hold a few tablespoons to scatter on top of the brownies before baking if desired).
Pour the batter into the baking tin, using a spatula to spread it out evenly. Slice the remaining bananas (if using) lengthways and arrange them on top of the batter.
Sprinkle the top of the bananas with coconut sugar and place in the oven to bake for 25-30 minutes. Check the brownies are done by piercing them with a clean knife or skewer, it should come out clean with a few crumbs. If there is a lot of batter on the knife, bake them for another 5 minutes.
Allow the brownies to cool completely before removing them from the pan and cutting them into servings.