Preheat oven to 375°F and generously grease two mini muffin tins or 1 12-cup muffin tin. Whisk together oil and sugar. Add eggs, vanilla and buttermilk and whisk or beat until well combined.
Toss together apples, 2 teaspoons of flour and 1/2 teaspoon cinnamon.
Stir in the diced apple. In a small bowl, whisk together the flour, baking powder, salt and cinnamon. Stir flour mixture into wet ingredients.
Divide batter evenly between 12 muffin cups or 48 mini muffin cups. For mini muffins bake for about 16 minutes, or for large muffins bake 30 minutes, or until lightly browned around the edges.
Remove to cooling racks. Combine the sugar and cinnamon in a small bowl. Brush the tops of the muffins with butter, then roll muffin tops in cinnamon sugar mixture. Serve warm. Best served the first day. Store leftovers in a covered container.
You can also make regular-sized muffins with this recipe. Bake at 350°F for about 30 minutes, or until a toothpick comes out clean. Be sure to check at 20-25 minutes to make sure they don’t overcook.