1. Preheat the oven to 350 degrees. Spray an 8 by 8 baking dish with cooking spray. Set aside.
2. To make the crisp, mix the flour, oats, brown sugar, cinnamon, and salt together in a medium bowl. Stir in the melted butter and mix until mixture is moist. Press half of the crisp topping into the prepared baking dish.
3. For the plum filling, put the plums, sugar, cornstarch, lemon juice, and vanilla in a large bowl. Stir until plums are well coated.
4. Spread the plums evenly over the bottom layer of the crisp. Sprinkle the remaining crisp over the plums. Gently pat the crisp topping down.
5. Bake in the middle of the oven for 45 minutes, or until the crisp is golden and the fruit is bubbling. Let cool for 20 minutes before serving.
*Note-this crisp is even better served with a scoop of vanilla ice cream.