Lightly grease a 9x13 cake pan.
Mix cake mix as on the package; add one stick of melted butter. Pour cake mix into the prepared pan. Pour rhubarb over the cake mix; pour sugar over the rhubarb. Slowly pour the cream over the sugar. Important, do not stir. Bake at 180ºC or 1 hour or until rhubarb is tender and dessert is bubbling.
Top with whipped cream, if desired.