Prune Whip

1 . Stage

Preheat the oven to 300 degrees F (150 degrees C). Butter and sugar a 2-quart baking dish.

2 . Stage

Combine prunes and 1/3 cup water in a medium saucepan over medium heat; bring to a simmer and cook until soft, about 5 minutes. Drain.

3 . Stage

Purée prunes in a blender, then transfer back to the saucepan. Add 1/3 cup sugar and heat until sugar dissolves. Remove from the heat and stir in lemon juice and vanilla.

4 . Stage

Beat egg whites in a glass, metal, or ceramic bowl until frothy. Add cream of tartar and continue to beat until stiff peaks form. Fold in prune purée. Pour mixture into the prepared baking dish.

5 . Stage

Bake in the preheated oven until nicely browned on top, 15 to 20 minutes. Cool to room temperature, then chill for at least 2 hours before serving.