Preheat the oven to 425°F. Line muffin tins with paper liners and set aside.
In a medium bowl, combine the flour, baking powder, baking soda, apple pie spice, and salt. Whisk together to combine.
In a large bowl, use a hand mixer to beat together the butter, oil, granulated sugar, and brown sugar until smooth and slightly fluffy.
Add the eggs and vanilla, mixing until fully incorporated.
Pour in the buttermilk and mix until just combined. The batter may look slightly curdled at this point; this is normal.
Add the flour mixture to the wet ingredients and mix until just combined. Do not overmix. Set aside.
In a small bowl, toss the finely diced apples, granulated sugar, and apple pie spice. Stir until the apples are well coated.
Gently fold the apple mixture into the muffin batter using a spatula, being careful not to overmix.
Divide the batter evenly among the muffin cups, filling each one to the top.
In a small bowl, mix together the granulated sugar and ground cinnamon for the topping.
Brush the tops of each unbaked muffin generously with melted butter, then sprinkle the cinnamon-sugar mixture evenly over the tops.
Bake the muffins for 18–20 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs but no wet batter.
Let the muffins cool in the pan for 5–10 minutes, then transfer them to a wire rack to cool completely.