Place butter in a 13- x 9-inch pan; place in a 350° oven for 5 minutes or just until melted. Remove from oven, and coat pan with cooking spray. Add brown sugar and cinnamon, stirring well. Spread mixture in pan.
Cut 3 bananas diagonally into 1/4-inch-thick slices. Arrange slices on brown sugar mixture in 4 lengthwise rows; cover and set aside.
Combine mashed banana and next 4 ingredients in a large bowl; beat at medium speed with an electric mixer until blended. Combine flour and next 4 ingredients; add to mashed banana mixture, beating on low speed just until blended. Pour batter evenly over sliced banana in pan.
Bake at 350° for 35 to 40 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 2 minutes; loosen cake from sides of pan with knife, and invert onto a large rectangular serving platter.
Drizzle crème de cacao over cake. Heat rum in a small saucepan just until simmering; ignite with a long match. Spoon rum over cake with a long-handled spoon. Serve warm cake with ice cream or whipped cream, if desired.