Make this delightful plum cake in late summer when the stone fruit is at its flavorful best. You'll want to use the ripest plums for the sweetest result. Made with plain yogurt, the cake is also extra moist and tender, and would be delicious dolloped with more yogurt (sweetened or not) on top of each slice when serving, either as an easy dessert or a special treat anytime of day (even breakfast!).