Soak the prunes in the apple juice or brandy for at least 30 minutes (overnight in case of brandy). Transfer them and the liquid into a small food processor and process until you get a purée. Set aside.
Preheat the oven to 180°C. Line a small baking tin with parchment paper.
Sift the flour, cocoa, salt and baking powder in a mixing bowl. Add the sugar.
Mix the eggs and the oil in another bowl, then add the vanilla. Pour this mixture into the flour mixture and combine. Add the prune purée and more apple juice (about 3-5 Tbsp) if needed and combine. You should get a smooth, thick batter - not too runny but not dough-like either.
Gently fold in the nuts, then transfer the batter into the lines tin, spreading it out evenly.
Bake in the oven for 25 minutes or so, until it’s golden - the middle should remain a bit gooey.
When it’s completely cooled down, cut it into squares (to avoid crumbs it’s best to put the cake in the fridge for 30 minutes or so before slicing).
Dust with icing sugar before serving.