Preheat oven to 350°F (175°C). Grease or line a 9x5-inch loaf pan with parchment paper.
In a large bowl, whisk flour, granulated sugar, baking powder, baking soda, salt, and cinnamon.
In another bowl, whisk melted butter, eggs, vanilla extract, and buttermilk until smooth.
Add wet ingredients to dry ingredients and stir until just combined.
Gently fold diced apples and nuts into the batter.
Pour half the batter into the loaf pan, sprinkle with half the brown sugar-cinnamon mixture, then add remaining batter and top with the rest of the cinnamon sugar.
Bake for 50–60 minutes, or until a toothpick comes out clean. Cover with foil if the top browns too quickly.
Cool for 10 minutes in the pan, then transfer to a wire rack to cool completely.
Whisk powdered sugar, milk, and vanilla until smooth. Drizzle over cooled bread.
Slice and serve warm or at room temperature.
