Toss together first 3 ingredients in a large bowl.
Melt butter in a large skillet over medium-high heat. Add plum mixture to skillet; bring to a boil, and cook, stirring often, 5 minutes. Spoon hot plum mixture into a lightly greased, shallow 2-quart baking dish. Spoon Spiced Plum Muffin Batter evenly over hot plum mixture.
Bake at 400° for 25 minutes or until golden brown.