I did that once many years ago. So, I crumbled the cake up, toasted it under the grill slightly. Then I put rhubarb that I had cooked into a dish, a little lemon zest over it, then the cake crumbs and some granulated sugar. I then finely grated a little butter on top and put it into the oven. Can't remember for how long. But, I took it out when it was golden brown on top, served it with double cream. You could also use vanilla ice cream. It was delicious!!!!! :o)