In a large mixing bowl, beat butter and shortening with a paddle attachment on low speed until creamy. Add sugar, milk, egg yolk, oil, baking soda, salt, and vanilla and beat again until blended. Add mashed bananas and flour alternately until smooth. Stir in 1/2 cup of the sifted flour until combined.
Spread half of the batter in prepared pan. Spread remaining batter on top and trowel gently onto sides of pan.
Bake at 350°F for 25 to 30 minutes or until a wooden pick inserted near center comes out clean.
Cool for 15 minutes on a wire rack. Remove from pan by lifting edge around edges with a metal spatula. Place upside down on a plate and let stand overnight.
Run a sharp knife along side of cake. Invert onto a serving platter. Using a spoon, drizzle melted cream cheese icing over the top. Dust lightly with powdered sugar and serve immediately.