Combine first 7 ingredients in a small saucepan; bring to a boil. Reduce heat, and simmer 6 minutes or until plums are tender and liquid is almost absorbed. Cool completely.
Preheat oven to 375°.
Lightly spoon flours into dry measuring cups; level with a knife. Combine flours, baking powder, baking soda, and salt; stir with a whisk. Place granulated sugar and butter in a large bowl; beat with a mixer at medium speed until well blended (about 4 minutes). Add egg; beat well. Add flour mixture; beat at low speed just until blended.
Divide dough into 24 portions. Roll each portion into a 2-inch circle on a lightly floured surface. Place 1 dough portion into a miniature muffin cup coated with cooking spray. Spoon about 1 tablespoon plum mixture into muffin cup. Repeat procedure with remaining dough balls and plum mixture.
Bake at 375° for 10 minutes or until lightly browned. Run a knife around outside edge of each tartlet; remove from pan. Cool completely on a wire rack. Sprinkle with powdered sugar.