Preheat oven to 350 degrees F. Butter 2 quart baking dish (9-inch x 9-inch glass baking dish works well here.)
Cream 1/2 cup butter and 1/2 cup sugar. Add in egg; mix well. Add flour, baking powder, milk, vanilla, and salt. Put batter into prepared baking dish, spreading evenly into dish.
Wash and quarter plums, lengthwise, discarding stems and seeds. Stand quartered plums in cake.
Mix the topping ingredients. Strew around top of cake, over and between plums. Bake 1 hour.
