Preheat the oven to 350°F. Use a paper towel to rub a little butter evenly on the bottom and sides of an 8½ x 4½-inch loaf pan.

In a mixing bowl, combine cassava flour, baking soda, baking powder, and salt using a wooden spoon or whisk.

Beat the bananas in a medium bowl until pureed and smooth.

Place butter and sugar in a large bowl and beat with an electric mixer on medium until pale yellow and creamy. Add eggs and vanilla and beat to combine well.

Add the banana puree and beat until smooth and well combined. Add the flour mixture and beat until smooth. Add the walnuts and mix at lowest speed until blended evenly through batter.

Transfer batter to prepared loaf pan and smooth the top. Bake 50-55 minutes or until edges have pulled away from the pan and a toothpick inserted into the center of the loaf comes out with just a few moist crumbs. Remove and cool 15 minutes in the pan. Turn onto a cooling rack and cool completely before serving.