Prepare ahead: Will keep for up to 1 week in an airtight container.

1. Preheat the oven to 350°F (180°C). Butter the loaf pan and line the bottom with wax paper. Sift the flour and baking powder together into a bowl. Add the butter and use your fingertips to rub it in until the mixture resembles bread crumbs. Stir in the sugar.

2. Mash the bananas with a fork'you should have 1 cup. Add the bananas, yogurt, and eggs to the flour mixture and stir until combined. Stir in the walnuts. Spread in the pan.

3. Bake for 1¼ hours, or until a wooden toothpick inserted in the center comes out clean. Transfer to a wire cake rack and let cool 5 minutes. Invert onto the rack, remove the paper, and cool completely.