Heat oven to 400°. Grease 12 medium muffin cups, 2 3/4x1 1/4 inches, with shortening; or line muffin cups with paper baking cups and spray with cooking spray.
Mix flour, cereal, brown sugar, baking powder, baking soda and salt in large bowl with spoon. Stir in egg, buttermilk and oil just until moistened. Gently stir in plums and walnuts. Divide batter evenly among muffin cups. Sprinkle 1/2 teaspoon of the granulated sugar on each muffin.
Bake 13 to 15 minutes or until toothpick inserted in center comes out clean. Remove from pan to wire rack. Serve warm.