For topping: Combine the flours, ¼ cup of sugar, baking powder, baking soda, and salt in a large mixing bowl. Whisk well, to mix. Add the butter and cut it in with a pastry blender until the mixture resembles coarse crumbs. In a small bowl, blend the buttermilk and vanilla. Make a well in the dry mixture and add the liquid all at once. Stir with a wooden spoon just until the dough is evenly mixed. Bring the fruit back to a boil on the stovetop, then turn off the burner. Spoon seven or eight equal dollops of dough over the fruit, close together.

Cover and bake on the low oven rack for 20 minutes. Do not check or uncover the pan. Transfer to a rack, remove the lid of the casserole, and sprinkle the top with sugar. Cool on a rack for at least 15 minutes before serving.