Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan or a 9-inch round springform pan. Set aside.
Step 2: In a medium bowl, whisk together the flour, baking soda, baking powder, cinnamon, nutmeg (if using), and salt. Set aside.
Step 3: In a large bowl, cream together the softened butter, granulated sugar, and brown sugar with an electric mixer until light and fluffy, about 2-3 minutes.
Step 4: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix on low speed until just combined, being careful not to overmix.
Step 6: Gently fold in the diced apples until evenly distributed throughout the batter.
Step 7: Pour the batter into your prepared baking pan and spread evenly.
Step 8: Bake for 45-50 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean or with a few moist crumbs attached.
Step 9: Remove the cake from the oven and let it cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely. Serve warm or at room temperature.