Preheat oven to 400°F (200°C) and grease a 12-cup muffin tin or line with paper liners.
In a bowl, mix flour, brown sugar, granulated sugar, and cinnamon for the topping. Add melted butter and mix until crumbly. Set aside.
In another bowl, whisk together flour, baking powder, cinnamon, and salt.
Toss diced apples with a small amount of the flour mixture to coat evenly.
In a large bowl, beat butter and both sugars until smooth and creamy (about 2 minutes).
Add eggs one at a time, mixing well after each. Stir in vanilla extract.
Gradually add dry ingredients and milk, alternating between them. Mix gently until just combined (do not overmix).
Fold in the coated apples evenly into the batter.
Divide batter into muffin cups (about ¾ full). Sprinkle crumb topping generously on top.
Bake for 10 minutes at 400°F (200°C), then reduce temperature to 350°F (175°C) and bake for 12–15 minutes or until a toothpick comes out with a few moist crumbs.
Let muffins cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
