Preheat your oven to 350°F (175°C) and line a 9×5-inch loaf pan with parchment paper, allowing overhang on the sides for easy removal.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
In a large bowl using an electric mixer, cream together the softened butter and sugar until light and fluffy, about 3-4 minutes.
Beat in the eggs one at a time, making sure each is fully incorporated before adding the next. Stir in milk, Greek yogurt, and vanilla extract until well combined.
Gradually fold the dry ingredients into the wet mixture, stirring just until combined. Do not overmix.
Fold in the diced apples, ensuring they're evenly distributed throughout the batter.
In a small bowl, combine brown sugar, cinnamon, and melted butter until it resembles wet sand.
Pour half of the apple batter into the prepared loaf pan, smoothing it into an even layer. Sprinkle about two-thirds of the cinnamon mixture evenly over the batter.
Add the remaining batter, then top with the remaining cinnamon mixture. Use a knife to gently swirl the layers together with a figure-eight motion, being careful not to overmix.
Bake in the preheated oven for 55-60 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. If the top browns too quickly, loosely cover with aluminum foil after about 35 minutes of baking.
Allow the bread to cool in the pan for 15 minutes, then use the parchment overhang to lift it out onto a wire rack to cool completely.
If desired, make the glaze by whisking together powdered sugar, milk or apple cider, and vanilla until smooth, then drizzle over the cooled bread.