In a large bowl, combine butter and sugar. Beat at medium speed with an electric mixer until fluffy. Beat in eggs, one at a time, beating well after each addition. Beat in bananas and vanilla.
In a medium bowl, combine flour, baking powder, baking soda, and salt. Add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Pour batter into prepared pan. Bake for 35 minutes, or until a wooden pick inserted in center comes out clean.
Let cool for 10 minutes. With the handle of a wooden spoon, poke holes halfway through cake in half-inch intervals.
In a medium bowl, combine pudding mix and milk. Whisk until smooth and slightly thickened, about 2 minutes. Pour over top of cake. Loosely cover cake and refrigerate for 2 to 4 hours.
Spread whipped topping over top of cake; sprinkle with grated chocolate, if desired. Cut into squares to serve.
•Use very ripe bananas for best flavor.
•Be sure to allow chilling time after topped with pudding--it needs at least 2 hours to set.
•Allow frozen nondairy whipped topping to thaw in refrigerator overnight before using. That ensures it will spread smoothly over cake.