Mix the flour, cocoa, sweetener and whey and mix well, then add the mashed banana and mix until it forms a dough (it looks like it won't mix and get homogeneous but it's kneading with a fork that the ingredients will mix with the banana), then add the yeast and stir gently.
Make cookie-shaped balls and place the chopped chocolate on top.
Bake in a preheated oven at 180 degrees for 10 minutes or in an airfryer preheated to 200 degrees for about 5 minutes.