Preheat oven to 375°F. Coat a baking dish with cooking spray. Place the flour, sugar, cinnamon, and salt in a large bowl and combine. Add 3 tablespoons of the butter and combine with a pastry cutter or 2 forks until it reaches a crumble. Add the nuts. Place the plums in the dish and top with the crumble. Dot with the remaining butter. Bake about 1 hour or until golden.