PREHEAT the oven to 350° F. Butter or lightly coat an 8 1/2-by-4 1/2-by-2 1/4-inch loaf pan with vegetable cooking spray.
WHISK together the flour, baking powder, baking soda, and salt, in a large bowl.
CREAM the butter and sugar until light and fluffy in a medium bowl. Add the eggs one at a time, beating well after each addition. Add the bananas, milk, and vanilla and stir with a wooden spoon or rubber spatula just until combined. The mixture may look curdled.
STIR in the flour and mix just until blended. Stir in the nuts, if using.
POUR batter into the pan and spread evenly. Bake for 45 to 55 minutes or until a cake tester or toothpick inserted in the center comes out clean and the bread is risen, lightly browned, and pulls away from the sides of the pan.
COOL the bread in the pan set on a wire rack for about 10 minutes. Remove the bread and let it cool completely on the wire rack before serving.
TIP: To store, put the bread in an airtight container or plastic bag and store at cool room temperature for up to 3 days. To freeze, wrap in plastic and then foil. Freeze for up to 3 months. Defrost at room temperature.