Coat 13×9-inch baking pan with vegetable cooking spray. Heat Dried Plum Purée and butter in Dutch oven or large saucepan over low heat, stirring until butter is melted. Add marshmallows; stir until completely melted. Remove from heat. Stir in cereal until well coated. Spray back of wooden spoon with vegetable cooking spray and pat mixture evenly into prepared pan. Sprinkle with nonpareils. Cool until set. Cut into squares.
Dried Plum Purée Combine 1-1/3 cups (8 ounces) pitted dried plums and 1/4 cup plus 2 tablespoons hot water in container of food processor or blender. Pulse on and off until dried plums are finely chopped and smooth. Store leftovers in a covered container in the refrigerator for up to two months. Makes 1 cup.