Preheat oven to 160C fan bake.
Lightly butter a 23 x 33cm baking pan and line with buttered baking paper.
Mix walnuts, brown sugar, cinnamon and salt then work in the butter with your fingertips until crumbly.
Set aside.
Combine flour, baking powder, soda and salt and set aside.
In a mixer, cream the butter for 2-3 minutes.
Gradually add sugar then eggs one at a time, mixing well. Beat in the sour cream and vanilla. Add flour mixture and beat until combined.
Fold in the apples and walnuts and pour into the baking pan.
Sprinkle the topping evenly over the cake. Bake for about 40 minutes until a knife inserted into centre comes out clean.
Remove from oven and leave to cool before cutting into squares to serve.
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