Preheat the oven to 350 degrees.
In a small bowl, mix together the brown sugar, flour, and cinnamon for the filling. Set that aside.
In a medium bowl, mix together the melted butter, brown sugar, flour, chopped walnuts, and cinnamon for the crumb topping. Mix well and set that aside.
In a large bowl or the bowl of an electric mixer fitted with the paddle attachment, add the butter and sugar. Mix well to combine.
Add the applesauce and sour cream. Mix again.
Crack the eggs into the bowl and mix well.
In a small separate bowl, add the flour, baking powder, and baking soda. Mix to combine.
Add the dry ingredients to the wet ingredients in the bowl of the stand mixer. Mix well until the flour mixture is completely mixed in, but do not overmix.
Butter a 9x13 inch baking dish well or spray with cooking spray. Pour half of the batter into the bottom of the prepare baking dish and spread it into an even layer. Sprinkle the filling mixture over the top of the batter. Top with the rest of the batter then sprinkle the walnut topping over the top of the cake.
Bake for 50-60 minutes or until a toothpick is inserted and comes out clean.