Preheat oven to 325°F. Spray a 9X9-inch baking pan.
Prepare the topping: Combine all ingredients and mix well. Set aside.
In a bowl, cream together butter, sugar and eggs. Add flour, baking soda, baking powder, salt, sour cream and vanilla; mix well.
Pour half of the batter into the pan. Layer with half of the apple slices, then half of the topping. Pour in the remaining batter, layer with the remaining apple slices, and then add the rest of the topping.
Bake for 1 hour and 15 minutes, or until a toothpick inserted in the center comes out clean. Baking time may vary depending on the oven, so take a peek at one hour and see how the cake looks. Let the cake cool and then refrigerate for at least 1 hour before serving.