Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round springform pan or a 9x13 inch baking dish.
Step 2: In a large mixing bowl, cream together the softened butter and granulated sugar until the mixture is light and fluffy.
Step 3: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, baking soda, salt, ground cinnamon, and ground nutmeg until well combined.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients, mixing just until combined and no streaks of flour remain. Be careful not to overmix.
Step 6: Gently fold the diced apples into the cake batter until they are evenly distributed.
Step 7: Pour the batter into your prepared pan and spread it evenly with a spatula.
Step 8: Bake for 45-55 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean.
Step 9: Let the cake cool in the pan on a wire rack for 10 minutes. Then, carefully invert it onto the rack to cool completely before slicing and serving.