Combine flour, almond flour, and the baking powder in a bowl
Add the powdered sugar through a strainer
Chop the butter (if it isn't already soft) and mash the banana with a fork
Add the butter and banana to the flour-mixture and stir/knead till combined
Let the dough rest in the fridge till cold (about 1 hour)
Divide the dough into two parts and roll each out to a square on a piece of baking paper
Divide each square into two parts of the same size with a butter knife
Bake on a baking pan in the middle of the oven at 175° C for 12-15 min till the dough has turned light brown
Let cool