Pre-heat the oven to 350F.
To an 8-inch square or round baking dish, add the plums, 3 tablespoons brown sugar, 1 tablespoon flour and ¼ teaspoon salt; toss everything together to coat; set aside.
In a medium mixing bowl, combine the graham cracker crumbs with the remaining ¼ cup flour, 2 tablespoons brown sugar and ¼ teaspoon salt.
In a small bowl, combine the butter and vanilla.
Using a spoon, mix the butter and vanilla mixture into the graham cracker mixture until the crumbs are evenly moist, then spread over the plums, pressing down with your palms to create an even, compact layer.
Place in pre-heated oven and bake about 30 minutes or until the crust is golden brown, hardened and plum juice is bubbling up along the edges of the pan.
Remove from the oven and let cool for at least 5 minutes.
Serve warm or at room temperature.
