Prepare your workspace. Preheat your oven to 350°F (175°C). Grease and line a 9×5-inch loaf pan with parchment paper, allowing some overhang on the sides for easy removal.
Mix the dry ingredients. In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg until well combined.
Cream the butter and sugars. In a large bowl, beat the softened butter, granulated sugar, and brown sugar with an electric mixer or by hand until light and fluffy, about 2-3 minutes. This incorporates air for a lighter texture.
Add the wet ingredients. Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Mix in the vanilla extract, then fold in the sour cream until just combined.
Combine wet and dry mixtures. Gradually add the dry ingredients to the wet ingredients, mixing on low speed just until no flour streaks remain. Overmixing will develop gluten and make your cake tough.
Fold in the apples. Gently fold the diced apples into the batter with a spatula, ensuring they’re evenly distributed throughout.
Transfer to the pan. Pour the batter into your prepared loaf pan and spread it evenly. If using, sprinkle the top with turbinado sugar and a light dusting of extra cinnamon.
Bake to perfection. Bake in the preheated oven for 55-60 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs (not wet batter).
Cool properly. Allow the cake to cool in the pan for 15 minutes, then use the parchment paper to lift it out onto a wire rack to cool completely before slicing.