Preheat Wolf oven on BAKE at 175°C and set a rack to position 3.
In mixing bowl, cream the butter and sugar until light and fluffy. Add eggs one at a time. Mix until combined. Sift together the dry ingredients. Alternate adding the dry ingredients with the sour cream and vanilla. Once mixed, add the apples and gently stir through. Pour the mixture into greased 9 x 13 pan.
For the topping, add all ingredients to bowl and mix with hands until crumbly. Sprinkle mixture on top of cake to form an even coating.
Bake in preheated Wolf oven for 35 to 40 minutes. When toothpick comes out clean from the center of the cake, remove and cool in pan. Best if served the next day.