Heat your oven to 170c/325f/GM3.
Cut 6 pieces of baking paper, line the muffin pan with the paper.
Put the banana and dates (removed from the water) in a jug. Blend with a hand blender until smooth (you can also use a food processor or smoothie blender for this).
Put the finely chopped walnuts, oats, gram flour, cinnamon and bicarbonate of soda into a mixing bowl. Mix well, add the banana/date mixture to the bowl, plus vinegar and water. Mix well, pour into the muffin pan quickly (the bicarbonate will start reacting to the vinegar immediately).
Put the remaining walnuts on top of the muffins. Bake the muffins for around 30 minutes until they are completely cooked through.
Serve either on their own or with a dollop of smooth peanut butter.