Preheat oven to 350°F. Spray pineapple pan with vegetable pan spray; add butter and brown sugar. Place pan in oven 5 minutes or until butter is melted, stirring once. Remove pan from oven to cooling rack; cool 2-3 minutes.
Slice one mango lengthwise into thin, fan-shaped strips. Cut remaining mango into 3/4-inch pieces. Arrange fan-shaped mango slices over sugar mixture in crown (leaf) portion of pineapple pan. Arrange remaining mango pieces in base portion of pan. Set pan aside.
Prepare cake mix according to package directions, adding rum extract, if desired. Carefully pour batter over fruit in prepared pan; spread mixture evenly. Bake 40-45 minutes or until toothpick inserted in center is clean when removed. Cool cake in pan on cooling rack 5 minutes. Gently loosen edge of cake around pan; turn cake out onto serving platter. Some topping may remain in pan; spoon topping evenly over warm cake. Best when served warm.