Chocolate chip banana muffins

Hi,
On Tuesday we made chocolate chip banana muffins. Some groups had to substitute one of the ingredients to something similar, but one group had to stick to the original recipe... my group replaced butter with oil. We made the mixture then put them in the oven for 10-15 minutes. When we took them out of the oven we let them sit for a little bit before we removed them from the tray. They tasted great but I think the bananas could have been mashed up a little more.