Follow these step-by-step instructions carefully to ensure your muffins turn out perfectly light, moist, and delicious every time. The key is to be gentle with the batter to avoid developing too much gluten.

1. Preparation and Preheating: Begin by preheating your oven to 400°F (200°C). This initial high heat helps the muffins rise quickly, creating those desirable domed tops. Place paper liners in a standard 12-cup muffin tin, or generously grease and flour the tin if you prefer to go without liners.
2. Combine the Dry Ingredients: In a large mixing bowl, add the all-purpose flour, granulated sugar, light brown sugar, baking powder, ground cinnamon, and salt. Use a whisk to stir everything together thoroughly for at least 30 seconds. This step is crucial as it not only combines the ingredients but also aerates the flour and ensures the baking powder and cinnamon are evenly distributed throughout. This prevents pockets of salt or leavener in your finished muffins.
3. Combine the Wet Ingredients: In a separate medium-sized bowl, crack the large egg and lightly beat it with a fork or whisk. Add the milk, melted butter, and vanilla extract to the beaten egg. Whisk these wet ingredients together until they are just combined and form a smooth, homogenous liquid.
4. Gently Combine Wet and Dry: Pour the entire mixture of wet ingredients into the large bowl containing the dry ingredients. Now, switch from a whisk to a rubber spatula or a wooden spoon. Gently fold the ingredients together until they are just combined. It is absolutely essential not to overmix the batter. The batter should look lumpy and thick, and there may still be a few small streaks of flour visible. This is perfectly fine. Overmixing develops the gluten in the flour, which will result in tough, dense, and rubbery muffins instead of light and tender ones.
5. Fold in the Walnuts: Add the chopped walnuts to the batter. Gently fold them in with your spatula, using just a few strokes until they are evenly incorporated. Again, be careful not to overmix at this stage.
6. Fill the Muffin Tin: Using an ice cream scoop or two spoons, carefully divide the batter evenly among the 12 prepared muffin cups. An ice cream scoop is highly recommended as it ensures each muffin is the same size, which means they will all bake evenly. Fill each cup about two-thirds to three-quarters of the way full. This will give them enough room to rise without overflowing.
7. Bake to Perfection: Place the muffin tin on the center rack of your preheated oven. Bake for 15 to 20 minutes. The muffins are done when they are golden brown on top and a wooden toothpick or skewer inserted into the center of a muffin comes out clean or with a few moist crumbs attached. If it comes out with wet batter, they need a few more minutes.
8. Cooling: Once baked, remove the muffin tin from the oven and place it on a wire cooling rack. Let the muffins cool in the tin for about 5 minutes. This allows them to set up slightly, making them easier to handle. After 5 minutes, carefully transfer the individual muffins from the tin directly onto the wire rack to finish cooling completely. Allowing them to cool fully on the rack prevents the bottoms from becoming soggy from trapped steam.