Preheat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together melted butter and sugar until smooth. Add eggs, milk, and vanilla; whisk until combined.
Add flour, baking powder, baking soda, salt, and cinnamon. Stir until just mixed. Gently fold in diced apples.
Divide the batter evenly among muffin cups.
In a small bowl, combine flour, brown sugar, and cinnamon. Cut in the cold butter with a fork or your fingers until you get pea-sized crumbles. Sprinkle evenly over the muffin batter.
Bake for 18–22 minutes, or until a toothpick comes out clean. Let cool slightly, then enjoy warm.