Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, cinnamon, and salt.
In another bowl, mix the melted butter, eggs, milk, and vanilla until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the diced apples until evenly distributed.
Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.
Bake for 20-25 minutes or until a toothpick inserted into the center comes out clean.
Remove the muffins from the oven and let them cool in the tin for 5 minutes.
Transfer the muffins to a wire rack to cool completely.
Enjoy your delicious apple cinnamon muffins warm or at room temperature!