Preheat oven to 450 degrees F (230 degrees C). Grease a baking sheet.
Combine sugar and cinnamon together in a bowl. Mix flour, oats, baking powder, 1 tablespoon cinnamon-sugar mixture, and salt together in a separate bowl. Cut butter into flour mixture using a pastry blender until mixture resembles coarse crumbs; fold in prunes.
Make a well in the center of the flour mixture; add milk to the well. Stir mixture with a fork just until dough is moistened. Drop dough into 12 mounds, about 1/4 cup each, on the prepared baking sheet. Sprinkle remaining cinnamon-sugar over each dough mound.
Bake in the preheated oven until biscuits are golden brown, 10 to 12 minutes. Serve immediately.
