Raspberry-Marshmallow Upside-Down Cake

The book 250 Classic Cake Recipes (1954) includes an entire section on upside-down cakes—everything from apple gingerbread to fruit mélange. Then I found one calling for rhubarb and marshmallows! I loved the crazy-sounding combo of fruit and marshmallows (the marshies caramelize around the fruit in the bottom of the pan in the most delicious of ways). My recipe replaces the rhubarb with raspberries, and I added more marshmallows than called fo