Preheat the oven to 375 degrees F (190 degrees C). Grease a 12 cup muffin tin or line with paper muffin cups.
In a large bowl, stir together 2 1/4 cups flour, baking soda and salt. In a separate smaller bowl, mix together the egg, buttermilk, 1/2 cup melted butter, vanilla and 1 1/2 cups of brown sugar until sugar has dissolved. Pour into the flour mixture and sprinkle the diced apple into the bowl as well. 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 and cinnamon. Drizzle in 2 tablespoons of melted butter while tossing with a fork until well blended. Sprinkle this over the tops of the muffins.
Bake for 25 minutes in the preheated oven, or until the tops of the muffins spring back when lightly pressed.