Preheat your oven to 350°F.
Line a muffin tin with cupcake liners.
In a large bowl, cream together your butter and sugar.
To your butter mixture, add the eggs, vanilla, and milk. Mix until everything is well combined.
Add your flour, baking powder, and cinnamon to your butter mixture. Mix until your batter is uniform and there are no large clumps of flour.
Gently fold your chopped apples into your muffin batter. Set the mixture aside.
In a smaller bowl, combine the remaining melted butter, brown sugar, flour, and cinnamon. The mixture should resemble a crumbly, wet sand.
Pour your muffin batter into your lined muffin tin before sprinkling each muffin with a generous handful of your crumb topping.
Bake your muffins at 350°F for 20 minutes or until a toothpick inserted into the center of one of your muffins comes out clean.
Allow your muffins to cool for a few minutes before serving and enjoying!