Step 1: Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease generously. Set aside.
Step 2: In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt. Make sure there are no lumps.
Step 3: In a separate medium bowl, whisk together the melted butter, granulated sugar, and brown sugar until well combined. Beat in the egg and vanilla extract until smooth.
Step 4: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined. Be careful not to overmix; a few lumps are fine.
Step 5: Gently fold in the chopped apples until evenly distributed throughout the batter.
Step 6: Divide half of the muffin batter evenly among the prepared muffin cups. Spoon about 1-2 teaspoons of caramel sauce on top of the batter in each cup. Cover with the remaining muffin batter.
Step 7: If desired, swirl the remaining caramel sauce (about 1/4 cup) on top of each muffin using a knife or skewer for a marbled effect.
Step 8: Bake for 22-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean. The tops should be golden brown and spring back when lightly touched.
Step 9: Let the muffins cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely. Drizzle with extra caramel sauce before serving, if desired.