Prepare the Batter

1. Heat the oven to 375 F and line a 12-well muffin tin with paper liners.
2. In a mixing bowl with an electric mixer, beat the butter and brown sugar until light and fluffy. Beat in the eggs until well blended. Beat in the vanilla.
3. In a separate bowl, combine the flour with the baking powder, cinnamon, and salt; stir or whisk to blend the dry ingredients.
4. Add one-third of the dry ingredients to the butter and brown sugar mixture and beat until blended. Add half of the milk and beat until blended. Repeat with another one-third of the dry ingredients and the remaining milk, then beat in the remaining flour until blended. Avoid overbeating.
5. Fold in the chopped apple and walnuts (if using).
6. Spoon the batter into the muffin cups, filling about 3/4 full.

Make the Crumble Topping

1. In a small bowl, combine the flour, brown sugar, and cinnamon. Work the butter into the mixture with a fork or your fingers until the crumbs are formed.
2. Sprinkle crumbs over each filled muffin cup.
3. Bake the apple muffins for 18 to 22 minutes until a toothpick inserted into the center of a muffin comes out clean.