Melt butter in a 9 by 1 1/2 inch cast iron frying pan and the add sugar. Place the rhubarb evenly in the pan.
Pour the cake batter over the rhubarb mixture.
Bake for 30 to 35 minutes, or until the cake is done.
Remove the cake from the oven and immediately run a spatula around the edge of the pan.
Invert onto a serving plate and let syrup drain onto the cake for a few
minutes before lifting the pan.