Preheat oven to 375°F.
In a bowl, cream together sugar and butter.
In another bowl, sift together flour, salt, cinnamon, and baking powder. Combine with butter mixture. Reserve ⅓ cup of mixture.
Press remaining batter evenly into bottom and about 1 inch up sides of a 10-inch pie plate or 8x8-inch baking pan. Arrange plums, rounded side up, in a single layer on batter. Sprinkle with reserved crumb mixture. Bake for 15 minutes.
In a separate bowl, beat egg and stir in cream. Pour over plums. Bake for 30 minutes more, or until custard is set. Serve at room temperature.