01 - Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease thoroughly.
02 - Whisk together the flour, baking powder, baking soda, and salt in a medium bowl.
03 - In a large bowl, using an electric mixer or whisk, beat the softened butter, granulated sugar, and brown sugar until light and fluffy. Blend in eggs one at a time, then add the vanilla extract.
04 - Add the dry ingredients to the butter mixture in three increments, alternating with buttermilk. Stir until just combined, ensuring not to overmix.
05 - Gently fold in the diced apples until evenly distributed throughout the batter.
06 - In a small bowl, mix together the brown sugar and ground cinnamon.
07 - Spoon half of the batter into the muffin cups. Sprinkle each with a pinch of cinnamon sugar filling, drizzle with a little melted butter and caramel sauce.
08 - Cover with the remaining batter and, if desired, top with more cinnamon sugar.
09 - Bake for 22 to 25 minutes or until a toothpick inserted in the center comes out clean.
10 - Let muffins cool in the pan for 10 minutes before transferring to a wire rack. Drizzle with additional caramel sauce before serving.