Preheat oven to 350°F. Line a muffin tin with paper or silicone liners, or lightly coat with cooking spray.
In a large bowl, combine the oil and sugar. Add the eggs, buttermilk, and vanilla extract, stirring until combined. Fold in the diced apple.
In a small bowl, whisk together the flour, baking powder, salt, and cinnamon.
Add the dry ingredients to the apple mixture and stir until just combined. Do not overmix the muffin batter.
Divide the batter evenly between the prepared muffin cups, filling each about ⅔ full.
Bake for 16-18 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with a few moist crumbs.
Remove from the oven and transfer the muffins to a wire rack to cool slightly.
In a small bowl, combine the sugar and cinnamon for the topping. Brush the muffin tops with melted butter, then dip each into the cinnamon sugar mixture until evenly coated.
Serve and enjoy!
Video
Notes
Dice the apple into small pieces so it distributes evenly throughout the muffins.
Avoid overmixing the batter, as this can make the muffins dense.
Granny Smith is my favorite apple for this recipe, but Honeycrisp, Pink Lady, Braeburn, or Fuji also work well.
Store cooled muffins in an airtight container at room temperature for up to 3 days or refrigerate for up to 5 days.
Freeze in a freezer-safe container for up to 3 months.