In a small bowl combine the spelt flour, baking powder, baking soda and salt. Mix thoroughly and set aside. In a larger bowl, stir together the oil, vanilla, and sugar. Stir until combined. Add the eggs, one at a time, and stir until fully incorporated. Add the milk, and again, mix well. Add the dry ingredients and mix just until completely combined. Pour the cake batter over the plums in the 8x8 baking dish. Wiggle the pan to remove any air bubbles and then place in the oven. Bake at 350 degrees for 40-45 minutes until the top is golden to dark brown and a toothpick inserted in the center comes out clean. Remove from the oven and cool at least 20 minutes before serving.