Preheat the oven to 350 F degrees. Line a 12-cup muffin-tin with paper liners and spray each liner with a bit of cooking spray. This will make it easier to remove the liner from the muffins.
In a medium bowl, combine the walnuts, brown sugar and ground cinnamon. Set aside.
In a large bowl, combine the flour, granulated sugar, and salt together. Add the cold butter to the bowl and using a pastry blender or two knives, cut the butter into the flour mixture until pea size. Remove 1 cup of this mixture and add it to the streusel walnut mixture and combine.
Add the baking powder and baking soda into the flour mixture and whisk to combine.
In a small bowl whisk together the apple sauce, egg and vanilla extract. Pour this applesauce mixture over the flour mixture and stir until just combined, do not over mix. Take 3/4 cup of the streusel mixture and add it into the muffin batter, and stir until combined. Gently fold in the chopped apples.
Add the 2 tbsp of melted butter to the streusel topping and stir until streusel resembles a crumble.
Divide batter among 12 muffin cups, I find that it's easier to do this using an ice cream scoop. Sprinkle generously with streusel topping, crumbling with your hands to make some big chunks. Bake for 25 to 30 minutes or until a toothpick inserted in the center of muffin comes out clean. Allow to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
While the muffins are baking you can make the glaze. In a small bowl add the icing sugar, milk, and vanilla extract, then whisk everything together. Drizzle over muffins.