Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners or lightly grease it to prevent sticking.
In a medium bowl, whisk together the unbleached all-purpose flour, baking soda, salt, and ground nutmeg until well combined. Set aside.
In a large mixing bowl, mash the ripe bananas until smooth. Add the granulated sugar, dark brown sugar, and melted butter, mixing well until combined. Pour in the milk and crack in the egg, whisking until the mixture is smooth.
Gradually add the dry ingredients to the wet mixture. Stir gently with a rubber spatula until just combined. Be careful not to overmix; a few lumps are perfectly fine!
Gently fold in the toasted and finely chopped macadamia nuts, ensuring they are evenly distributed throughout the batter.
Using a spoon or an ice cream scoop, fill each muffin cup about ¾ full with the batter. This allows room for the muffins to rise beautifully.
Place the muffin tin in the preheated oven and bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the muffins from the oven and let them cool in the tin for about 5 minutes before transferring them to a cooling rack. Enjoy warm or at room temperature!