Preheat the oven to 375 degrees. Grease a 12 cup muffin tin or line the tin with paper liners.
Place the flour, baking soda and salt into a large bowl, stir with a fork or whisk the ingredients to combine them.
Place the egg, buttermilk, 1/2 cup melted butter, vanilla and 1 1/2 cups brown sugar in a medium bowl. Stir the ingredients together until the sugar is dissolved and not laying on the bottom of the bowl.
Pour the buttermilk mixture into the flour mixture and sprinkle the diced apples into the bowl. Stir just until the mixture is blended .
Spoon the batter into the muffin tins, filling the cups to the top. Sprinkle the top of each muffin with muffin topping. Bake at 375 degrees for 25 minutes in a preheated oven or until the tops of the muffins spring back when lightly pressed. Remove from the oven to a wire rack to cool.