Drain the chickpeas, retaining at least 150 ml of the liquid (aquafaba). Use the chickpeas later in a salad or stew.
Using the whisk attachment on a hand mixer, beat the retained aquafaba (150 ml) with the sugar, vanilla and salt until fluffy but not stiff. Melt the chocolate with the coconut oil in a bain-marie. Allow to cool slightly, stir into the fluffy mixture. Mix in the flour, baking powder and nuts. Transfer the brownie mixture to the prepared tin.
Approx. 35 mins. in the centre of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, remove the tin frame, leave to cool completely, cut into approx. 25 squares.