Preheat the oven to 180c/350f.
In a large bowl, mash the banana, then stir in the nut butter.
Add the flour and baking powder and stir to combine, then use your hands to knead into a large mass.
Pinch off around 1 tablespoon of the mix and roll into a ball in your hands, then flatten to around 0.5cm in thickness and place on a lined baking tray/sheet. Repeat for 16-20 cookies.
Use your thumb to press a dent in to the tops of each of the cookies, if they spilt around the sides just gently press them back together.
Add 1/2-1 tsp of chia jam to each thumbprint hole.
Place in the oven and cook for 9-10 minutes until lightly golden around the edges.
Allow to cool on the tray for a few minutes and then transfer to a cooling rack to cool completely.