Preheat oven to 180 degrees C. Grease a 23cm baking tin.
Sift together the flour, baking powder, salt and cinnamon then set aside.
In a large bowl, mix together melted butter, brown sugar and caster sugar with a wooden spoon until smooth. Stir in the egg and vanilla. Blend in the flour mixture until just combined, then stir in the apples and walnuts.
Spread the mixture evenly into the prepared baking tin. In a cup or small bowl, stir together the remaining cinnamon and sugar then sprinkle over the top.
Bake for 25 to 30 minutes in the preheated oven. When cooked the slice should spring back when lightly touched. Cool in the tin then cut into squares or rectangles.