Preheat oven to 350 degrees. Brush a 9-inch square cake pan or 10-inch cast iron skillet with 2 tablespoons butter; set aside. Whisk together flour, 3/4 cup sugar, the baking powder, allspice, and 3/4 teaspoon salt in a medium bowl; set aside.
Whisk together egg, milk, vanilla, and remaining 4 tablespoons butter in another medium bowl. Add egg mixture to flour mixture; stir to combine. Spread batter evenly into buttered pan.
Toss plums, nectarines, lemon juice, remaining 1/4 cup plus 2 tablespoons sugar, and a pinch of salt in a large bowl. Spread fruit mixture evenly over batter. Sprinkle with topping. Bake until a cake tester inserted into center comes out with moist crumbs, about 1 hour and 15 minutes. Let cool in pan on a wire rack 1 hour before serving.