Follow these step-by-step instructions to bake your own batch of moist and fluffy apple cinnamon muffins:

1. Preheat the Oven: Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or lightly grease with cooking spray.
2. Prepare Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, ground cinnamon, and nutmeg. Set aside.
3. Cream Butter and Sugar: In a large mixing bowl, use an electric mixer to cream the softened butter and granulated sugar together until light and fluffy, about 2-3 minutes.
4. Add Wet Ingredients: Add the eggs one at a time, beating well after each addition. Then mix in the vanilla extract and sour cream until fully combined.
5. Combine with Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix, as this can lead to dense muffins.
6. Fold in Apples: Gently fold in the diced apples with a spatula, ensuring they are evenly distributed throughout the batter.
7. Fill Muffin Tin: Divide the batter evenly among the prepared muffin cups, filling each about ¾ full.
8. Prepare Topping: In a small bowl, mix together the brown sugar and ground cinnamon. Sprinkle this mixture over the top of each muffin for a sweet, crunchy topping.
9. Bake: Place the muffin tin in the preheated oven and bake for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
10. Cool and Enjoy: Allow the muffins to cool in the tin for about 5 minutes before transferring them to a wire rack to cool completely. Serve warm or at room temperature.