Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it with cooking spray.
In a large bowl, whisk together the flour, granulated sugar, baking powder, cinnamon, and salt until well combined.
In another bowl, mix the melted butter, milk, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Do not overmix.
Fold in the diced apples and nuts (if using) until evenly distributed.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
In a small bowl, combine the brown sugar and cinnamon for the topping. Sprinkle this mixture evenly over the muffin batter in the cups.
Bake for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.