Preheat and Prepare: Preheat the oven to 350°F. Grease and line a 9-inch round cake pan with parchment paper. A 9-inch springform pan also works. Make sure to grease the sides well and line the bottom with parchment.
Mix Dry Ingredients: Whisk the flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a bowl.
2 cups all-purpose flour, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt, 1 teaspoon ground cinnamon, ¼ teaspoon ground nutmeg
Cream Butter and Sugar: Beat the butter and sugar until light and fluffy.
½ cup unsalted butter, ½ cup granulated sugar
Add Eggs and Liquid: Mix in the eggs one at a time, followed by the buttermilk and vanilla.
2 large eggs, ½ cup buttermilk, 1 teaspoon vanilla extract
Combine and Fold in Apples: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in the diced apples.
3 medium apples
Mix Cinnamon Sugar: In a small bowl, combine the cinnamon and sugar.
2 tablespoons granulated sugar, ½ teaspoon ground cinnamon
Top and Bake: Pour batter into the pan, smooth the top, and sprinkle with cinnamon-sugar mixture. Bake for 25-35 minutes or until a toothpick inserted in the center comes out clean.
Cool: Let cool in the pan for 10 minutes before transferring to a wire rack.