1 Preheat the oven to 350 degrees.2 Beat the butter and sugars together until smooth. Mix in the egg and vanilla extract.3 Finely chop the banana (there will be about 3/4 cup) and stir it into the sugar mixture.4 Stir in the oats, flour, salt, baking powder and walnuts.5 Place the cookies about 1 1/2 inches apart on parchment lined cookie sheets.6 Bake until light brown, about 15 minutes.