Preheat oven to 180C/350F. Butter and line the base and sides of a 7cm deep, 20-22 cm wide brownie tin/square cake tin with baking paper.
Place chopped dates and bicarb soda in a bowl. Pour over boiling water. Allow to stand for 10 minutes or so to soften.
Using an electric mixer, beat butter, sugar and vanilla until pale and creamy. Add aquafaba in a few stages, beating well until combined and fluffy.
Sift over flour, baking powder, cinnamon and salt. Fold through using a large spoon, then fold the date mixture through until just combined.
Spoon mixture into prepared cake pan. Bake for 35 to 45 minutes, or until a skewer inserted into the centre comes out with no raw batter attached (it may not be clean thanks to the dates!).
Allow to cool for 5 minutes in the pan, then turn out onto a serving plate.