Turn on your oven to 350°F (175°C) now, so it’s fully heated when it’s time to bake.
Place the flour, baking soda, salt, and cinnamon in a large bowl and whisk together until smooth and uniform. This dry mix sets the stage for all the flavor.
Melt the butter gently and let it cool slightly, warm, not hot. While that’s cooling, mash your bananas in a separate bowl until smooth.
Add in the melted butter, vanilla, sugars, egg, and the milk. Stir all the ingredients until they come together into a smooth mixture.
Pour the wet mixture into the dry ingredients and gently fold until just combined. Don’t stir too much, having a few lumps is completely normal. Fold in most of the chopped walnuts, saving some for the tops.
Coat or line your muffin tray, then distribute the batter evenly, filling each cup to around three-quarters capacity. Sprinkle the rest of the walnuts over the top for a nice crunch.
Bake for 24 minutes. Let them sit in the pan for some time, then move them to a wire rack to cool. Or enjoy one warm!