If anyone can get rhubarb this custard cake in a 9x13 pan is great. I have started my own rhubarb patch to have plenty to make this great cake. Am thinking you might also use the scratch recipe given above by Jo Anne B and add the sugar and rhubarb.

https://www.tasteofhome.com/recipes/...-custard-cake/

I tried the recipe posted by SuzzyQ. Decided to make the pudding and leave it on the bottom of the pan and put the cake batter over it. I added 4 T. cocoa to the pudding and cooked it in my 8" sq. pan in the microwave. I then mixed the cake part and poured over the pudding and baked it not quite thirty minutes. The pudding did run over a little in the corners so a bit larger pan wouldn't have hurt. I had a piece for my lunch while it was still warm and thought it was delicious. I will have to pass this by my sister in law to see if it is as she remembers. Whether it is or not makes no difference. It is yummy. Thanks again for posting. I do have a similar recipe for rhubarb made the same way with the water poured over the top. I would be glad to share if anyone is interested.

CORRECTION!!!!
I checked with a family member concerning the Chocolate Lush cake recipe.
There is 3/4 Cup Sugar in the syrup mix.

The correct portions:
3-4 Tbs Cocoa
3/4 Cup sugar
1 and 3/4 Cup Hot water.

So sorry for the confusion and big THANK YOU to the person who got me to thinking/worrying and alerted me to my error.
Jo

Thank you for posting the correction. We all make mistakes. The only person who has never made a mistake is a person who has never done anything. They say we learn from our mistakes. According to that, I should be pretty smart by now. Ha! Ha! Happy cooking and baking!