Preheat your oven to 375F.
Line a 12-cup muffin tin with paper muffin liners and set aside.
In a large mixing bowl, combine the all-purpose flour, baking powder, baking
soda, salt, and cinnamon.
In another bowl, whisk together the melted butter and sugar until well combined.
Beat in the eggs, one at a time, followed by the milk and vanilla extract.
Gradually add the wet ingredients to the dry ingredients, stirring just combined.
*Be careful not to overmix at this stage.
Once the batter reaches a uniform consistency, go ahead and fold in the diced
apples.
To make the crumble topping, in a small bowl, combine the brown sugar, flour,
cinnamon, and melted butter.
Mix with a fork until it resembles coarse, pea-sized crumbs.
Divide the batter evenly among the prepared muffin cups.
Sprinkle each muffin with a bit of the crumble topping.
Bake at 375F for roughly 20-25 minutes, or until a toothpick inserted into the
center of a muffin comes out clean.
Remove from the oven, and allow the muffins to cool in the pan for 15 minutes at
room temperature.
Carefully transfer the muffins to a wire rack to cool completely.