Preheat your oven to 350°F (175°C). This ensures that the muffins bake evenly and rise properly.

In a large mixing bowl, whisk together the dry ingredients: flour, baking powder, baking soda, cinnamon, and salt. This helps to evenly distribute the leavening agents.

In another bowl, combine the melted butter and sugar, mixing until well combined. This creates a creamy texture that helps to incorporate air into the batter.

Add the eggs one at a time, whisking well after each addition. Then stir in the vanilla extract and milk until the mixture is smooth.

Gradually fold the dry ingredients into the wet mixture using a spatula. Be careful not to overmix; a few lumps are okay as this keeps the muffins tender.

Gently fold in the diced apples and walnuts (if using) until evenly distributed throughout the batter.

Grease the muffin tin or line it with muffin liners to prevent sticking.

Scoop the batter into the prepared muffin tin, filling each cup about 2/3 full. This allows room for the muffins to rise without overflowing.

Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. This indicates that the muffins are fully baked.

Once baked, remove the muffins from the oven and let them cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.