Heat oven to 350 F.
Grease and line an 8 x 8 inch square baking pan with a piece of parchment paper.
In a small bowl, combine all of the ingredients for the cinnamon sugar filling.
In a medium bowl, stir together the melted butter, brown sugar, flour, chopped walnuts and cinnamon to form the crumble topping. Set aside.
In the bowl of an electric mixer using the paddle attachment or in a large bowl using a hand mixer, cream butter and sugar.
Add the applesauce and sour cream to the butter-sugar mixture, followed by the egg. Mix well to combine.
Sift the dry ingredients (flour, baking powder, and baking soda) into the wet ingredients. Combine flour mixture and wet ingredients, stirring until no streaks remain.
Pour half the batter into the bottom of the prepared pan. Sprinkle the filling mixture over the top of the batter. Top with the rest of the coffee cake batter, followed by the walnut crumb topping.
Bake 50-60 minutes until the top of the cake is golden brown and until a toothpick inserted in the middle of the cake comes out clean.
Remove to a wire rack and allow to fully cool before slicing into 9 squares.
This easy coffee cake with a tender crumb keeps well wrapped tightly in plastic wrap, or stored in an airtight container.