To make crust: Use electric beaters or a large whisk to beat egg whites in a large bowl until stiff. Beat in egg yolks, then sugar and melted butter. Dissolve yeast in 1/2 cup warm water; stir into egg mixture. Sift flour and salt; stir into egg mixture. Turn dough onto floured surface; knead gently until no longer sticky, 1 minute or less. Cover and let rise in warm place until doubled in size, about 1 hour.

Meanwhile, make filling and topping: Simmer prunes in water 4 minutes; drain. Coarsely puree prunes with remaining filling ingredients in a food processor or blender. Set aside to cool. Whisk topping ingredients in separate bowl until smooth.

To bake pies: Butter 3 standard eight-inch, or 2 nine-inch deep-dish, pie pans. Punch down dough, divide into balls, and place in pie pans. Cover and let rise 10 minutes. Heat oven to 350 degrees. Pat dough in each pan to cover bottom and sides, with small edge around rim. Let rise 5 minutes. Spread filling in crusts. Pour topping in center of filling and spread it to within 1 or 2 inches of edge. Bake until crusts are lightly browned and topping is set, 20 to 30 minutes. Cool and serve in wedges.