1. Weigh or lightly spoon flours and potato starch into dry measuring cups; level with a knife. Combine flours, potato starch, baking powder, xanthan gum, 1/2 teaspoon salt, and baking soda in a large bowl, stirring with a whisk. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Make a well in center of flour mixture, and slowly add buttermilk, stirring with a wooden spoon until combined. Chill until ready to use.
2. Combine plums, 1/4 teaspoon salt, sugar, and remaining ingredients in a large bowl. Pour into a 10- to 12-inch skillet and place over medium-high heat. Bring to a boil, cover with lid, and decrease heat to medium-low. Cook until liquid begins to thicken, about 15 minutes, stirring occasionally.
3. Drop dough by spoonfuls, 2 tablespoons each, onto plum mixture. Cover pan, and cook 15 minutes.