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Prune Filling
1/4 pound prunes 2 tablespoons gelatin 4 tablespoons cold water 1/2 cup sugar 1/2 cup rhubarb or canned pineapple-juice 1/2 cup whipped cream Wash the prunes, soak overnight in water to cover, and cook slowly until soft. Remove pits and rub pulp through a coarse sieve. Soak the gelatin in cold water. When soft, add it to the boiling prune pulp and stir until the gelatin dissolves. Add sugar and fruit-juice. When the filling has cooled, fold in the whipped cream.
A gelatin-set fruit filling of the sort that held together a layer cake without cooking a custard. It works, but the gelatin quantity needs judgment before you commit.
- The gelatin is the hingeTwo tablespoons of powdered gelatin (roughly two envelopes) will set about two cups of liquid to a firm, sliceable jelly. Here it goes into roughly two cups of pulp, juice, soaking water and cream, so if you follow the original expect something firm enough to cut cleanly and chewy on the tongue rather than a soft filling; reduce the gelatin if you want a softer set.
- Soften, do not boilGelatin loses strength if held at a hard boil. Stir it into the hot pulp off the heat and keep stirring until no grains remain on the back of the spoon.
- Prune quantityA quarter pound of dried prunes is about 3/4 cup whole; the sieved pulp will come to rather more than that, depending on how much water the fruit takes up in soaking and cooking. Modern pitted prunes skip the pit-removal step and the overnight soak can be shortened to an hour in hot water.
- Rhubarb juiceNot something you are meant to buy ready-made here. Stew chopped rhubarb stalks with a little water until collapsed and strain off the liquid; use only the stalks. Canned pineapple juice, the alternative given, is the easier route and adds acidity that brightens the prune.
- SievingA coarse sieve and a wooden spoon give the texture intended, with some body left in the pulp. A blender turns it to smooth puree, which sets differently and tastes flatter.
- Cool before the creamFold the whipped cream in when the mixture is cool but not yet set -- about the consistency of unbeaten egg white. Too warm and the cream melts; too cold and the filling breaks into lumps as you fold.
- Yield and assemblyEnough to fill one two-layer 8- or 9-inch cake. Because it is gelatin-set and contains cream, the filled cake needs refrigeration and is best eaten within a day.
Prunes were a staple of the American pantry in the 1920s and '30s -- cheap, shelf-stable, sweet, and promoted heavily by California growers who had planted vast acreages of drying plums. Cookbooks of the period put them everywhere: in whips, in souffles, in bread, and in fillings like this one. The gelatin here reflects the other great convenience of the era, powdered gelatin sold in measured envelopes, which let a home cook stabilize whipped cream and fruit pulp into something that would sit between cake layers without soaking them. The pairing of prune with rhubarb or pineapple juice is a practical one rather than a traditional one: prune pulp alone is cloying, and the acid cuts it.