For cake: Preheat the oven to 350°F. Grease and flour a 13x9x2-inch baking pan. In a large bowl, mix together the oil and sugar. Beat in the eggs and vanilla and set aside. In another bowl, sift together the flour, baking powder, baking soda, and salt. Stir in the poppy seeds. Add the dry ingredients to the oil mixture alternately with the sour cream, beginning and ending with the dry ingredients. Fold in the apples and pour the batter evenly into the baking pan.

For topping: Combine walnuts, sugar, cinnamon, and butter. Mix until crumbly and sprinkle over the batter. Bake for 35 to 45 minutes, or until a toothpick inserted in the center comes out clean. Cool in the pan for 10 minutes.