Preheat oven to 350 degrees and line a muffin tray with paper or silicone liners (or cooking spray).
Combine the oil and sugar together in a medium bowl.
Add in the eggs, buttermilk, and vanilla extract.
Stir in the diced apple.
In a small bowl, combine the flour, baking powder, salt, and cinnamon.
Combine the dry ingredients with the apple mixture and stir until just combined.
Divide batter evenly amongst the prepared muffin tray(s), filling about 2/3 full.
Place pan in preheated oven and bake 16-18 minutes.
Remove from oven and place on a cooling rack.
Combine the sugar and cinnamon together to create the topping.
Brush the melted butter evenly on top of the muffins and dip into the cinnamon/sugar mixture.
Serve immediately and enjoy!