First, start by preheating the oven to 375F and fill a muffin tin with 10 large muffin liners.
In a large bowl, combine the flour, cinnamon, nutmeg, baking powder, and salt and set aside.
Using a paddle attachment, cream the butter with the brown sugar and granulated sugar for about 2-3 minutes. Then add in the ripe bananas. Don't worry about mashing them, the paddle attachment will do that for you.
Add in the large egg and vanilla, and mix until well combined. This step should take 30-60 seconds.
Combine the dry ingredients and the toasted walnuts into the wet ingredients and stir until just combined, do not over mix!
Scoop the batter evenly into the 10 large muffin liners, sprinkle with the remainder of the walnuts and some granulated sugar and bake for 24-27 minutes or until the muffins are golden brown and a tooth pick inserted in the middle comes out clean.
Let the muffins cool down in the muffin tin for 5 minutes then transfer them to a wire rack to cool some more.
Store these delicious banana muffins in an airtight container in the fridge for up to 7 days or in the freezer for up to 2 months