Preheat the oven to 180°C fan and line an 8-inch square baking tin with baking paper.
Whisk the aquafaba and cream of tartar (if using) in a stand mixer or a large bowl with electric beaters until it starts to become fluffy. Gradually add the caster sugar whilst whisking until all the sugar has been incorporated and the mixture is thick, glossy and looks like stiffly beaten egg whites.
120 ml aquafaba, 1/4 tsp cream of tartar
In the same pot you were using earlier, melt the chocolate and vegan butter over a low heat, stirring occasionally to prevent it catching. Lastly, stir in the water and remove from the heat.
150 g 70% chocolate, 100 g vegan butter, 100 g water
In a separate bowl mix the flour, almonds, cocoa powder, cornflour, bicarbonate of soda and salt.
100 g plain white flour, 70 g ground almonds, 50 g unsweetened cocoa powder, 30 g cornflour, 1/4 tsp bicarbonate of soda, 1/4 tsp fine table salt
Pour the melted chocolate into the bowl of whisked aquafaba and sift in the flour mixture. Fold together until combined and pour into your prepared tin.
Take spoonfuls of the maple caramel and dot over the surface of the brownie batter.
Bake for 25-30 minutes until the top looks dry but the centre is still soft. Remove from the oven, sprinkle with the flaky salt and allow to cool completely before slicing into 16 squares.
Store in an airtight container for up to 1 week.