Preheat oven to 350°F (175°C) and grease loaf pan. Line with parchment.
Whisk flour, baking soda, salt, cinnamon, and pistachios in a bowl.
Mash bananas, then mix with butter, sugar, egg, vanilla, and milk.
Combine wet and dry gently. Do not overmix.
Pour into pan, top with pistachios, and bake 55–60 minutes.
Cool 10 min in pan, then transfer to wire rack. Rest before slicing.