Fudgy Brownies

Ingredients

• 2 cup all-purpose flour
• 1 teaspoon baking powder
• pinch of salt
• 1 banana, peeled and mashed
• 1¼ cup superfine sugar
• 1 tablespoon vanilla extract
• 5½ ounce good-quality dark chocolate (minimum 70 percent cocoa solids), broken into pieces
• 4 tablespoon vegan margarine, plus extra for greasing
• 1 tablespoon agave nectar
• 1/3 cup dark chocolate chips
• 1 cup walnuts, chopped
• confectioners sugar, to dust