Preheat your oven to 350°F (175°C). Lightly grease a 9×5-inch loaf pan, then line it with parchment paper, allowing some overhang for easy removal.
In a small bowl, mix the brown sugar and cinnamon until completely combined. Set this aside for layering later.
Peel and dice your apples into small, roughly 1/4-inch pieces.
In a large bowl, cream together the butter and granulated sugar until light and fluffy – about 3-4 minutes. Add eggs one at a time, beating well after each addition. Mix in the vanilla extract until just combined.
In a separate bowl, whisk together the flour and baking powder. Add this dry mixture to the wet ingredients alternating with milk, beginning and ending with the flour mixture. Mix until just combined.
Pour half of the batter into your prepared loaf pan. Sprinkle half of the apple pieces over the batter, then sprinkle with half of the cinnamon-sugar mixture.
Pour the remaining batter over the top, followed by the remaining apples and cinnamon-sugar mixture.
Using a knife or skewer, gently swirl through the batter to create that signature fritter-like ribbon of cinnamon and apples.
Bake for 50-60 minutes, or until a toothpick inserted in the center comes out clean. If the top begins to brown too quickly, loosely cover with aluminum foil after about 30 minutes of baking.
While the bread is cooling, make the glaze by whisking together powdered sugar, milk, and vanilla until smooth. Start with 1 tablespoon of milk and add more as needed to achieve your desired consistency.
Once the bread has cooled in the pan for 15 minutes, use the parchment paper overhang to lift it out onto a wire rack. Let it cool for another 10 minutes before drizzling with the glaze. Allow the glaze to set for 5 minutes before slicing.