You can use any kind of berries — fresh or frozen — for this jammy twist on the classic pineapple upside-down cake. Just note that adding all or mostly blueberries and strawberries will create a sweeter topping than mixing in more acidic blackberries and raspberries. Caramelizing the brown sugar in a skillet before adding the fruit gives you a particularly deep, complex flavor. Because of the moisture in the topping, you’ll need to bake this cake a little longer than other, similar butter cakes. You want the surface to be well browned all over, with dark brown edges that yield a slight crunch. Underbaked cake will be soggy, and apt to fall apart.