Preheat your oven to 350°F (175°C) and prepare a 9x5-inch loaf pan by greasing it with butter or a non-stick cooking spray.
In a large mixing bowl, sift together the flour, granulated sugar, baking powder, baking soda, salt, ground cinnamon, and ground nutmeg.
In a separate bowl, combine the melted butter, buttermilk, eggs, and vanilla extract. Whisk until well incorporated.
Gradually pour the wet mixture into the dry ingredients. Stir gently until just combined.
Carefully fold in the diced apples and the optional chopped nuts.
In a small bowl, combine the brown sugar with an additional teaspoon of cinnamon.
Pour half of the prepared batter into the greased loaf pan. Sprinkle half of the brown sugar mixture over the batter. Pour in the remaining batter and finish with the remaining brown sugar mixture. Swirl through the batter for a marbled appearance.
Place the loaf pan in the preheated oven and bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for about 15 minutes, then transfer to a wire rack to cool completely.
Dust the top with powdered sugar before serving.
Notes
For an elegant finish, dust with powdered sugar and serve with fresh apple slices.