Preheat the oven to 350°F (175°C). Grease a 9 x 13 inch baking pan and set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon until well combined.
In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, about 2-3 minutes.
Add the eggs one at a time, beating well after each addition.
Mix in the sour cream and vanilla extract until smooth.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the diced apples gently.
Pour the batter evenly into the prepared baking pan and spread it out smoothly.
In a small bowl, combine the brown sugar, chopped walnuts, and melted butter to make the streusel topping. Mix until crumbly.
Sprinkle the streusel topping evenly over the batter in the pan.
Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and allow the cake to cool in the pan on a wire rack for at least 20 minutes before slicing and serving.