Preheat oven to 350°F (175°C). Grease an 8.5x4.5-inch or 9x5-inch loaf pan and line with parchment paper with overhang for easy removal.

In a medium bowl, whisk flour, baking powder, and salt. In a small bowl, mix the brown sugar and cinnamon for the swirl.

In a large mixing bowl, beat softened butter and granulated sugar for 2 minutes until light and fluffy. Add eggs one at a time, followed by vanilla extract.

Alternate adding dry ingredients and milk in two batches, mixing on low speed just until combined (do not overmix). Gently fold in the finely diced apples.

Spread half of the batter evenly into the prepared loaf pan.

Scatter half of the cinnamon brown sugar mixture evenly over the batter layer.

Spoon the remaining batter on top and spread gently. Sprinkle the remaining cinnamon brown sugar over the top. Use a butter knife to gently swirl through the batter in a figure-eight pattern. Drizzle melted butter and turbinado sugar over top.

Bake at 350°F for 50 to 55 minutes, until a wooden toothpick inserted into the center comes out clean or with a few moist crumbs.

Cool in the pan for 15 minutes, then use parchment handles to lift the loaf onto a wire cooling rack. Allow to cool for at least 30 minutes before slicing into thick slices.