Preheat the oven to 350 degrees F. Grease an 8-inch baking tin and line with parchment paper.
Sift the flour, baking powder, and salt into a large bowl.
In a medium bowl, combine the mashed banana, sugar, and vanilla extract.
Bring a small pan of water to the boil and then turn down to barely a simmer. Put a heatproof bowl on top, place the chocolate, margarine, and agave in the bowl and melt together until smooth and glossy. Pour the melted chocolate into the banana mix and stir to combine.
Make a well in the center of the flour, pour in the chocolate mixture and fold thoroughly until everything is incorporated. Finally, fold in the chocolate chips and walnuts, ensuring not to overwork the batter.
Pour into the prepared tin and bake in the oven for 25 minutes.
Let cool completely before cutting into pieces and dusting the brownies with confectioners sugar.
