Step 1

Preheat oven to 180C. Brush an 18cm square shallow cake pan with butter to lightly grease. Line the base and 2 opposite sides with a strip of non-stick baking paper, allowing the ends of the paper to overhang the sides.

Step 2

Combine the sugar, cocoa powder, apple, walnuts, egg whites, oil and vanilla essence in a medium bowl, and use a wooden spoon to mix well.

Step 3

Sift the flour and baking powder together over the cocoa mixture. Use the spoon to mix gently until well combined.

Step 4

Spoon mixture into pan and bake in preheated oven for 30 minutes or until a skewer inserted into centre comes out clean. Set aside for 5 minutes before turning onto a wire rack to cool. Dust with cocoa if desired.