Mash the banana well with a fork. Beat butter and sugar until foamy, stir in flour, baking powder, banana, egg and cinnamon. Fold the walnuts into the dough and use a spoon to place around ten piles on two baking sheets with baking paper.
Bake in a preheated oven at 150 degrees Celsius for about ten minutes. The cookies should still be a little soft when they come out of the oven.