Preheat your oven to 350°F (175°C) and prepare a bundt pan by generously greasing and flouring it to ensure easy release. In a medium bowl, beat the softened butter and sugar together until the mixture becomes light and creamy. Incorporate the eggs one at a time, followed by the vanilla extract, beating until smooth and well combined.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, alternating with additions of the apple sauce and sour cream. Mix until all ingredients are just blended; avoid overmixing to maintain a light texture.

For the streusel, mix the brown sugar, white sugar, cinnamon, chopped apple, and walnuts in a bowl until evenly combined. Spoon half of the batter into the prepared bundt pan, smoothing it out evenly. Sprinkle half of the streusel mixture over the batter layer, then gently swirl a knife through the streusel to create a marbled effect, pressing it slightly into the batter.

Add the remaining cake batter, followed by the rest of the streusel, ensuring an even and generous top layer. Bake the cake in the preheated oven for 40-45 minutes, or until a knife or toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for a few minutes before inverting it onto a serving plate. Let it cool completely before slicing to enjoy the rich apple cinnamon flavors combined with a delightful crunchy streusel topping.