Preheat the oven to 350°F (175°C) and prepare a Bundt pan by greasing it well.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the eggs, one at a time, mixing well after each addition, then stir in the vanilla extract.
In another bowl, combine the flour, baking powder, baking soda, cinnamon, and nutmeg.
Gradually add the dry mixture to the buttery batter, alternating with the buttermilk and mixing until just combined.
Fold in the diced apples and walnuts (if using) gently until evenly distributed.
Pour the batter into the prepared Bundt pan, smoothing the top with a spatula.
Bake in the preheated oven for 50-60 minutes or until a toothpick inserted comes out clean.
Once baked, let the cake cool in the pan for 15 minutes before inverting onto a wire rack.
Dust the cooled cake with powdered sugar before serving for a charming finish.