Preheat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line it with parchment paper.
Peel and finely dice the apples. Toss them with granulated sugar, cinnamon, and flour. Set aside.
In a medium bowl, whisk together flour, baking powder, salt, and cinnamon.
In a large bowl, beat the softened butter and granulated sugar until light and creamy.
Add the eggs one at a time, mixing well after each addition. Stir in vanilla.
Gradually add the dry ingredients, alternating with the milk. Mix just until combined.
Spread half of the batter into the prepared loaf pan.
Add half of the cinnamon apples and sprinkle with some of the brown sugar-cinnamon mixture.
Add the remaining batter and spread gently.
Top with the remaining apples and cinnamon mixture.
Drizzle the melted butter over the top.
Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool in the pan for about 15 minutes before transferring to a wire rack.
For the glaze, whisk powdered sugar, milk, and vanilla until smooth.
Drizzle the glaze over the cooled bread before slicing and serving.