Preheat the oven to 190℃ (170℃ fan)/375°F /gas mark 5). Line a muffin tin with 11 cases, or grease well and dust with flour.
Place 125g (4oz) plain flour, 125g (4oz) wholemeal flour, 125g (4oz) sugar and 2 teaspoon baking powder in a large bowl. Stir to combine, then add 50g (2oz) chcolate chips to the bowl.
Mash 2 large or 4 small bananas with a fork and add to the bowl.
Beat together 125ml (4floz) milk and 2 eggs with a fork then beat in 4 tablespoon tahini. Then stir in 75g(3oz) melted butter. Pour into the centre of the dry ingredients.
Stir with a spoon until the ingredients are just combined. Do not over beat the mixture, it should be lumpy at this stage.
Spoon into the muffin cases, filling almost to the top. Sprinkle with sesame seeds.
Bake for 20 to 25 minutes or until the muffins are risen and golden. Allow to cool in the tin for a few minutes then transfer to a wire rack until ready to serve.