Preheat your oven to 350ºF and line a muffin tin with muffin papers.
Prepare the streusel topping by combining oats, flour, brown sugar, walnuts, butter, and vanilla extract in a mixing bowl. Use your hands or a spoon to create a crumbly texture. Set aside.
In another bowl, whisk together flour, baking powder, salt, and cinnamon. Set aside.
Cream together brown sugar, vanilla extract, and butter until fluffy using an electric mixer.
Mix in applesauce until combined, then add eggs and blend.
Incorporate dry ingredients into the wet mixture until smooth. Fold in diced apples.
If the batter is too thick, add milk until it reaches the desired consistency.
Fill muffin cups ¾ full and top each with 1-1.5 tablespoons of streusel, gently pressing it into the batter.
Spray streusel with cooking spray and bake for 18-22 minutes until cooked inside.