Preheat the oven to 350ºF and line a muffin tin with muffin papers.

Prepare the streusel topping. Add the rolled oats, flour, brown sugar, walnuts, butter, and vanilla extract to a mixing bowl. Use a spoon (or your hands work best) and cut the ingredients together until a crumble forms. This may take 4-5 minutes. Set streusel topping aside for later.

Next, add the flour, baking powder, salt, and ground cinnamon to a bowl and whisk the ingredients together. Set aside.

In a separate bowl, cream together the brown sugar, vanilla extract, and butter until light and fluffy with an electric mixer.

Add the apple sauce to the creamed mixture and mix until combined. Add the eggs and mix to combine. Add 1/2 cup of milk to the batter.

Next, add the dry ingredients to the wet ingredients and mix until smooth. Fold the diced apples into the batter. If the consistency is thick and creamy, your batter is ready. If your batter is too thick, like cookie dough, add 1 tablespoon of milk at a time until it reaches a thinner consistency.

Fill each muffin about ¾ of the way full, and then top each muffin with 1-1.5 tablespoons of streusel over the top of each muffin. Gently press the streusel into the top of the muffin batter. Spray the streusel with a little bit of cooking spray and place the muffins in the oven.

Bake the muffins for 18-22 minutes or until they are cooked on the inside.