Preheat your oven to 375 degrees Fahrenheit (190 degrees Celsius). Prepare a 12-cup standard muffin tin. You can grease the cups or line them with paper muffin liners.

1. Get a large mixing bowl. Add the flour, baking soda, and salt. Stir these dry ingredients together until they are combined.
2. In a separate bowl, which can be smaller, put the brown sugar, buttermilk, the half cup of melted butter, the egg, and the vanilla extract. Whisk this mixture well until the sugar seems dissolved.
3. Pour the liquid mixture from the smaller bowl into the large bowl with the dry ingredients.
4. Gently sprinkle the diced apple pieces over the mixture in the large bowl.
5. Stir everything together just until the ingredients are barely combined. Be careful not to overmix the batter.
6. Spoon the batter into the prepared muffin cups. Fill each cup all the way to the top.
7. Take a small bowl. Combine the half cup of brown sugar, the third cup of flour, and the cinnamon in this bowl. Stir them together.
8. Drizzle in the two tablespoons of melted butter over the dry topping ingredients.
9. Use a fork to mix the butter into the dry ingredients. Toss with the fork until the mixture is well blended and crumbly.
10. Sprinkle this topping mixture evenly over the tops of the filled muffin cups.

Bake the muffins in the preheated oven for about 25 minutes. The muffins are done when their tops spring back gently when you lightly press them.