Coat a 1 1/2-quart souffle dish with cooking spray; sprinkle with 1 tablespoon sugar, and set aside. Combine 2 tablespoons sugar, Prune Puree, and chocolate in a bowl; stir and set aside.
Beat egg whites, cream of tartar, and salt at high speed of a mixer until soft peaks form. Gradually add remaining 2 tablespoons sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently fold one-fourth of egg white mixture into prune mixture; gently fold in remaining egg white mixture. Spoon into prepared souffle dish.
Place souffle dish in a 9-inch square baking pan; add hot water to pan to a depth of 1 inch. Bake at 350° for 55 minutes or until puffed and set. Sprinkle with powdered sugar. Serve immediately.