Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan.

In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set this dry mixture aside.

In another bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes.

Add the eggs one at a time to the butter mixture, mixing in each egg fully before adding the next. Stir in the vanilla extract.

Alternately add the dry ingredients and buttermilk to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined.

Fold in the diced apples gently to avoid breaking them. Be careful not to overmix the batter.

In a small bowl, combine the cinnamon and 1/4 cup of granulated sugar to create the cinnamon swirl mixture.

Pour half of the batter into the prepared loaf pan and sprinkle half of the cinnamon sugar mixture over it. Add the remaining batter and top with the remaining cinnamon sugar mix.

Using a knife, swirl the batter gently to create a marbled effect, but do not overmix. This creates your beautiful cinnamon ribbon.

Bake for 55-65 minutes, or until a toothpick inserted in the center comes out clean. Let it cool in the pan for about 10 minutes before transferring it to a wire rack.

To make the glaze, mix powdered sugar with milk until smooth. Once the bread is completely cooled, drizzle the glaze on top.