Preheat the oven to 350 degrees. Grease a 12 cup muffin tin or line with paper muffin cups.
In a large bowl, combine 2 1/4 cups flour, baking soda, & salt (or use self rising flour).Beat in the egg, buttermilk, melted butter, vanilla and 1 1/2 cups of brown sugar until well combined. Stir in the diced apples. Stir just until everything is blended. Spoon into the prepared muffin tin, filling the cups to the top.
In a small bowl, stir together 1/2 cup of brown sugar, 1/3 cup flour, cinnamon, & melted butter.. Sprinkle this over the tops of the muffins.
Bake for 25-30 minutes in the preheated oven, or until the tops of the muffins spring back when lightly pressed.