In a 9-inch square baking pan (or an 11x7-inch pan) sprayed with non-stick spray, stir flour, nuts, sugar, cornstarch, baking soda, and salt with a fork. Add bananas, oil, egg, orange juice, and vanilla. Mix with a fork until blended.
Bake at 350 degrees for 30-35 minutes, or until cake pulls away from sides of pan and toothpick inserted in center come out clean. Cool in pan. Spread with orange frosting.
Orange Buttercream Frosting
Cream butter with an electric mixer. Gradually add sugar, beating well until combined. Add orange juice, orange peel and vanilla; stir to blend. Spread on cooled cake