Heat oven to 180C/160C fan/gas 4. Cream the butter and sugar together until well blended and smooth. Gradually beat in the banana, vanilla extract and egg. Add the zest. Mix well with a wooden spoon until thoroughly blended.
In a large bowl, mix the flour, salt, baking powder, oats, coconut, walnuts and sultanas or chocolate chips. Stir the dry ingredients into the wet and mix thoroughly until a thick dough is formed. Line a baking sheet with parchment. Drop heaped tbsps onto the sheet, leaving a 5cm (2in) space around each one, and press down lightly. Bake for 15-20 mins until lightly browned. Cool.