Rhubarb buttermilk upside down cake with orange and cardamom

Woah what a mouthful of a title. and what a mouthful of a cake. You can just call it breakfast - like I did this morning and eat 3 slices! Treacly sharp rhubarb on a sweet, tender cake fragrant with cardamom. Combining three ideas: one from Darina Allen, one from Sarah Raven and an adapted buttermilk cake recipe from Rose Levy Beranbaum's wonderous Cake Bible.