Preheat the oven to 180°C (fan) and line two large, flat baking trays with greaseproof paper.
Take the cookie dough out of the fridge.
Scoop up heaped tablespoons of cookie dough and use your hands to roll them into balls. Place on the baking trays and do not flatten. You will need to leave space between each cookie so they have room to spread out. I'd suggest baking 4-6 cookies on a baking tray at a time and bake them in batches. Just keep the cookie dough in the fridge in-between batches so it doesn't get too soft or sticky*.
Bake for 10-12 minutes. The cookies should be golden on the outside but pale and soft in the middle. Don't worry if the middle still looks under-baked, this is how they are meant to be and the cookies will carry on baking as they cool down.
Whilst the cookies are still warm, push the leftover milk chocolate and walnut chunks into the top of each cookie and use a round cutter to scoot them into shape.
Leave the cookies on the baking tray to cool down and firm up for 15 minutes. This will improve the cookie texture so try to resist the temptation to tuck in straight away.
Store your banana bread cookies at room temperature, in an airtight container, for 2-3 days.