A depression-era dessert that remained popular through the 60s. Here’s a reference from the Feb. 24, 1933 edition of The (St. Petersburg) Evening Independent in an article titled “Prunes No Longer Humble But Attract Attention of High-Browed Epicure.”
From a box sold in Canby, Oregon.
Baked Prune Whip
2 c. cooked prunes
1 tsp. grated lemon peel
2 tsp. lemon juice
4 Tbsp. confectioners sugar
4 stuff beaten egg whites
Mash prunes. Add lemon peel and juice and 2 Tablespoons sugar. Blend.
Add remaining sugar to egg whites; pile lightly on baking dish. Bake 350 deg. 20 to 30 minutes.
Combine 3 egg yolks, dash salt, 1/4 c. sugar–stir in 2 cups milk (scalded) cook in double boiler until thick. Remove from heat. Add 1 tsp. vanilla. Chill.