Preheat the Oven Preheat your oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan, or line it with parchment paper for easy release.
Prepare Dry Ingredients In a medium bowl, whisk together flour, baking soda, baking powder, cinnamon, and salt. Set aside.
Cream Butter and Sugar In a large mixing bowl, beat softened butter and sugar until light and fluffy (about 3–4 minutes). This aeration step helps create a tender, moist loaf.
Add Eggs and Vanilla Beat in eggs, one at a time, then add vanilla extract. Mix until smooth and creamy.
Combine Wet and Dry Ingredients Gradually add the dry mixture, alternating with buttermilk. Begin and end with dry ingredients. Mix just until combined — overmixing can make the loaf dense.
Fold in Apples Gently fold in diced apples using a spatula. You want the fruit evenly distributed but not crushed.
Make the Cinnamon Swirl In a small bowl, combine brown sugar, cinnamon, and melted butter into a thick paste.
Layer the Batter Pour half the batter into your prepared loaf pan. Spoon half the cinnamon mixture on top and swirl gently with a knife. Add the remaining batter, top with the remaining swirl, and create another light swirl pattern.
Bake the Loaf Bake for 55–65 minutes, or until a toothpick inserted in the center comes out clean. If the top browns too quickly, tent it with foil.
Cool and Glaze Let the loaf cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.If using the glaze, whisk together powdered sugar, milk, and vanilla. Drizzle over the cooled loaf before serving.
