01 - Preheat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease lightly.
02 - In a large bowl, whisk together flour, baking powder, baking soda, salt, and 1 1/2 teaspoons of cinnamon.
03 - In a separate bowl, whisk together melted butter, sugar, eggs, milk, and vanilla extract until smooth.
04 - Add the wet ingredients to the dry ingredients. Stir gently until just combined. Avoid overmixing.
05 - Fold in the diced apples until evenly distributed throughout the batter.
06 - Divide the batter evenly among the prepared muffin cups, filling each approximately three-quarters full.
07 - Combine the turbinado sugar and 1/2 teaspoon of cinnamon for the topping, then uniformly sprinkle it over the batter in each muffin cup.
08 - Bake for 20-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
09 - Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.