Preheat oven to 350F, then grease and line a 9x5-inch loaf pan. Keep the pan ready so the batter goes in immediately after mixing.

Mix brown sugar and ground cinnamon, then toss the apples in 2 tbsp of the mixture. The apples should look evenly coated with specks of cinnamon sugar.

Beat softened butter and granulated sugar until fluffy. The mixture should lighten in color and look smooth.

Beat in eggs one at a time, then mix in vanilla extract. Stop when the batter looks glossy and fully combined.

Whisk all-purpose flour, baking powder, and salt together. Make sure there are no visible lumps of baking powder.

Add the dry ingredients to the batter alternately with milk, mixing just until no dry streaks remain. The batter should be thick and spoonable.

Spread half the batter in the loaf pan. Level the top so the first apple layer sits evenly.

Top with half the apples and half of the remaining cinnamon sugar, then spread the rest of the batter over. Aim for a fairly even second layer.

Repeat the apple and cinnamon sugar layers, then gently swirl with a knife. You should see marbled ribbons of cinnamon sugar throughout the loaf.

Bake at 350F for 50-55 minutes, until a toothpick comes out clean. Look for a golden top and set edges.

Cool the loaf for 15 minutes before glazing. The loaf should hold its shape when touched lightly.

Whisk powdered sugar with 2-3 tbsp milk until smooth, then drizzle over the loaf. Finish when the glaze forms a thick ribbon that slowly drips down the sides.