Line a couple of cookie sheets with baking parchment paper.
In a large mixing bowl measure out the flour and add the unsalted butter (cut up in to small pieces). Using your hands, work the butter into the flour until the butter gets absorbed in the flour.
Add the peanut butter and work it into the flour, either with your hands or a wooden mixing spoon.
Add the sugar, pinch of salt and chocolate chips and mix well together.
In another bowl, mash the banana with a fork.
Add the egg and mix it into the mashed bananas.
Add milk to the banana mixture and mix well together.
Add the wet ingredients to the flour ingredients and mix them together. There is no need to cream or overmix the dough, just bring it all together. You should get a fairly soft dough.
Using a desert spoon, scoop the dough and drop on your baking tray, making sure that you leave good space between each cookie. Use more trays if needed as the cookies will spread a little.
At this point you could choose to chill the trays with the ready cookies for 30 minutes or so. This prevents them from loosing their shape when they are baking. It's not completely necessary, but it helps especially in the summer or if your kitchen is very warm.
Place in the middle of your oven and bake for 15 minutes or until the cookies are nicely turning golden with a lightly brown edges.
Using the greaseproof paper, lift your cookies from the baking tray and slide them on the cooling rack. Leave to cool down completely.