Step 1: Preheat your oven to 400°F (200°C) and line a 12-cup muffin tin with paper liners or grease with non-stick spray.
Step 2: In a small bowl, prepare the streusel by mixing 1/2 cup flour, brown sugar, and cinnamon. Cut in the cold butter with a fork or pastry blender until the mixture resembles coarse crumbs. Set aside in the refrigerator.
Step 3: In a large bowl, whisk together the flour, granulated sugar, baking powder, cinnamon, and salt for the muffin batter.
Step 4: In another bowl, combine the melted butter, maple syrup, milk, egg, and vanilla extract. Whisk until well combined.
Step 5: Pour the wet ingredients into the dry ingredients. Stir gently with a spatula until just combined. Do not overmix.
Step 6: Gently fold in the diced apples until they are evenly distributed through the batter.
Step 7: Scoop the batter into the prepared muffin tin, filling each cup about 3/4 full.
Step 8: Generously sprinkle the chilled streusel topping over each muffin.
Step 9: Bake for 5 minutes at 400°F, then reduce the heat to 350°F (175°C) and bake for an additional 15-20 minutes, or until a toothpick inserted into the center comes out clean.
Step 10: Allow the muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.