Pour scalded milk over the butter, sugar, and salt in a bowl; set aside.
Soften yeast in the warm water.
Add about 1/2 cup flour to milk mixture and beat until smooth.
Stir in yeast.
Add about half of the remaining flour and beat until very smooth.
Beat eggs with lemon peel until thick; blend into mixture.
Mix in enough of the remaining flour to make a soft dough.
Turn onto a lightly floured surface.
Cover and let rest 5 to 10 minutes.
Knead dough until smooth and satiny.
Form into a ball and put into a greased deep bowl.
Turn dough to bring greased surface to top.
Cover; let rise in a warm place until doubled, about 1 hour.
Punch down dough and turn onto a lightly floured surface.
Divide dough into halves.
Press each evenly into a lightly greased (bottom only) 9-inch layer cake pan.
Brush dough with melted butter or margarine.
Arrange plums, cut side up, on dough to 1 inch from edges of pans.
Sprinkle with a mixture of 1 1/2 to 2 cups sugar, 2 tablespoons flour, and 2 teaspoons ground cinnamon.
Dot with softened butter or margarine (about 1/2 cup).
Cover; let rise again until almost doubled, about 45 minutes.
Bake at 375°F 15 minutes.
Beat together 4 egg yolks, 2 tablespoons sugar, and 1/4 cup cream; then pour over the plums.
Continue baking 15 to 20 minutes, or until custard is set.