Preheat oven to 350°F Coat a 9-inch springform pan or a 13 x 9-inch baking dish with cooking spray.
To prepare cake, beat butter until fluffy. Add sugar and beat well. Add eggs, one at a time, and vanilla; beat well. Add flour, cinnamon and salt; beat until just combined. Stir in banana and cooked oatmeal.
Spread batter in pan. Bake about 35 minutes. Remove cake from oven. Heat broiler.
To prepare topping, melt butter in saucepan. Add milk and brown sugar; stir until sugar dissolves. Add oats, coconut and walnuts; stir well. Spread over warm cake. Place under broiler and broil until browned and bubbly, about 4 minutes.