jfddrsn -- I remember the mix you are writing about - but I haven't seen it in years.
This recipe is from James Beard's AMERICAN COOKERY - published about 1972. I HAVE NOT made this, but it does have the hot water poured over - which makes the cake end up on top and the custard/pudding on the bottom. You might want to try it, if it sounds like what you remember. If so, please let us know how it turns out!
LEMON CAKE PUDDING
3 well-beaten egg yolks
3 tablespoons flour
1 cup sugar
1/2 teaspoon salt
1 tablespoons melted butter
6 tablespoons freshly squeezed lemon juice
1 tablespoon grated lemon rind
1-1/4 cups milk
3 egg whites, stiffly beaten
Combine the egg yolks with the dry ingredients which have been sifted together. Add melted butter; lemon juice, lemon rind and milk. Lastly fold in the stiffly beaten egg whites. Pour into a 2=quart baking dish and place in 350 degree F oven for 40 minutes until firm. The trick of the pudding is that it separates into a sponge-like cake on top and the custard/pudding rests on the bottom. Very good served with whipped cream.