Preheat oven to 350°F (175°C) and grease a 9x5-inch loaf pan.
In a medium bowl, whisk together flour, baking soda, salt, and cinnamon.
In a large bowl, beat melted butter and sugar until smooth. Add eggs and vanilla, mixing well.
Stir in buttermilk, then gradually add the dry ingredients until just combined.
Fold in diced apples and pour the batter into the loaf pan.
In a small bowl, mix brown sugar and cinnamon, then sprinkle over the batter.
Bake for 50-55 minutes, or until a toothpick inserted in the center comes out clean.
Let the bread cool in the pan for 10 minutes before transferring to a wire rack to cool completely.