Preheat your oven to 350°F (175°C). Grease and flour an 8.5 x 4.5-inch loaf pan thoroughly.
In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set the dry mixture aside.
In a separate large mixing bowl, use an electric mixer to beat the softened butter, granulated sugar, and vanilla extract on medium-high speed. Continue beating until the mixture becomes light and fluffy, around 3 to 4 minutes, occasionally scraping down the sides of the bowl.
Incorporate the eggs one at a time, mixing well after each addition until fully blended.
With the mixer on low, gradually add half of the dry flour mixture, followed by half of the milk. Continue alternating with the remaining flour mixture and milk, ensuring all ingredients are well combined.
For the apple mixture, combine the chopped apple pieces, ground cinnamon, and brown sugar in a separate bowl. Stir until the apples are well coated, then set aside.
To prepare the topping, mix together the brown sugar and ground cinnamon in a small bowl. Set this mixture aside for later use.
Pour half of the batter into the prepared loaf pan. Evenly spread half of the apple mixture over the batter. Using a knife or skewer, gently swirl through the batter without over-mixing to create a marbled effect.
Add the remaining batter on top, followed by the rest of the apple mixture. Swirl again lightly with a knife. Sprinkle the prepared cinnamon-sugar topping over the surface.
Bake in the preheated oven for 45 to 55 minutes. The bread is done when a toothpick or cake tester inserted in the center comes out mostly clean, with a few moist crumbs attached.
Allow the bread to cool in the pan for 15 to 20 minutes. In the meantime, prepare the glaze by whisking together the confectioners' sugar, water, and cinnamon.
Remove the bread from the pan and transfer it to a cooling rack. Drizzle the cinnamon glaze over the warm bread. Let it cool completely before slicing.