Cake: Preheat oven to 165 degrees C. Grease and flour two 20cm round cake tins.
Mix together the flour, bicarb soda and salt. Set aside. In a large bowl, whisk together the oil, sugar, eggs and vanilla. Stir in the flour mixture then fold in the apples and walnuts.
Pour batter into prepared cake tins. Bake in the preheated oven for 25 minutes or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.
Icing: In a saucepan melt the margarine and brown sugar over medium heat. Stir in the evaporated milk. Bring mixture to a boil then cook for 3 minutes stirring constantly. Remove from heat. Allow mixture to cool.
To Assemble: Take the cooled icing then fill and ice cake layers. Sprinkle with walnuts.