This recipe is designed with optimal ingredients and methods to ensure beautiful, tall muffins. If your muffins grew nicely in the oven, then sank as they cooled, there are a few things that may have gone wrong. 1) Too much leavening agent (baking powder or baking soda). 2) Over-mixed batter. Remember to gently incorporate wet and dry ingredients. 3) The muffins were not cooked all the way through yet.