Preheat oven to 350°F (175°C). Grease a 9-inch loaf pan.
In a small bowl, mix brown sugar and 1 teaspoon cinnamon. Set aside.
In a large bowl, beat butter and sugar until creamy. Add eggs one at a time, then mix in vanilla.
Stir together flour and baking powder. Add to the batter, alternating with milk, and mix until smooth.
Toss diced apple with 1 tablespoon sugar and 1 tablespoon cinnamon.
Pour half the batter into the loaf pan. Top with half the apples and half the brown sugar mixture. Lightly press down.
Add remaining batter, then top with remaining apples and brown sugar mixture. Gently swirl with a knife.
Bake for 40–50 minutes, or until a toothpick inserted in the center comes out clean. Cool completely.
Whisk glaze ingredients until smooth and drizzle over cooled bread.
Enjoy!