Preheat oven to 175°C (350°F) and lightly spray or oil a 20x20cm baking dish with oil.
Place apple purée in a medium-sized bowl. Sift in cocoa, flour and baking soda. Add sugar and salt and mix until just combined. Do not over-mix as this will toughen the brownies. Gently fold in chocolate chips and walnuts. Transfer to prepared baking dish and bake for about 25-30 minutes or until the centre feels set and fudgy when a skewer is inserted.
Cool in dish for 5-10 minutes before turning out. Cool completely before slicing into squares. Dust with sifted icing sugar before serving if preferred.