In a large bowl, combine the flour, sugar, baking powder and salt. Stir in milk and egg just until blended. Divide into six portions.
On a lightly floured surface, pat each portion of dough into a 5-in. circle. Place a plum half on each circle. Gently bring up corners of dough to center; pinch edges to seal.
In a Dutch oven, bring water and butter to a boil. Carefully add dumplings. Reduce heat; cover and simmer for 20-25 minutes or until a toothpick inserted into a dumpling comes out clean. Serve warm with pan juices, melted butter and cinnamon-sugar.