In a small bowl, toss the diced apples with brown sugar and cinnamon. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy.
Beat in the eggs and vanilla extract.
Add flour and baking powder, alternating with milk, mixing just until smooth.
Pour half the batter into a greased loaf pan. Top with half the apple mixture. Repeat with remaining batter and apples.
Use a knife to gently swirl the layers.
Bake at 350°F for 50–60 minutes, or until a toothpick inserted in the center comes out clean.
Let the bread cool completely.
Mix powdered sugar with 1–2 tbsp milk until smooth and drizzle over the cooled bread.