Preheat your oven to 350°F (175°C) and grease a loaf pan.
In a large bowl, mix together the flour, sugar, baking powder, salt, and cinnamon.
In a separate bowl, whisk the melted butter, eggs, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Fold in the diced apples gently to avoid breaking them.
Pour half of the batter into the prepared loaf pan.
In a small bowl, mix brown sugar and cinnamon, then sprinkle half of this mixture over the batter.
Pour the remaining batter on top and sprinkle with the rest of the cinnamon-sugar mixture.
Bake for 45-50 minutes or until a toothpick comes out clean.
Let it cool in the pan for 10 minutes before transferring to a wire rack.