Mash the banana in a large mixing bowl until relatively smooth. It should take 3 average sized ones, but spoon the mashed ‘nanas into a measuring cup to make sure, then return to the mixing bowl. Beat in the sugar, olive oil, milk, vanilla and rosemary.
Sift in the half the flour, and all the baking soda and salt and gently mix. Fold in the chocolate chips. Add the remaining flour and mix to incorporate, making sure not to overmix.
Lightly grease an 8×4 loaf pan. Pour the batter into the pan. If you’d like to decorate, place the banana cut sides up lenthwise over the batter. Scatter a few springs of rosemary over the top.
Bake for 55 minutes. It should be lightly browned and pulling away from the sides. The chocolate chips and banana topping can make it look underdone when you test it, so go by the sides pulling away from the pan and the top being lightly golden."