Using the back of a wooden spoon, poke at least 3 rows of 4 holes in the cooked cake, making 12 holes total.
In a large bowl, mix 2 mashed bananas, 1 cup of the melted peanut butter, and the sweetened condensed milk; if needed, add extra milk to loosen the mixture. Pour the mixture over the cake and let it settle into the holes for about 10 minutes. Some of the mixture will stay on top, which is okay.
Spread the whipped topping evenly over the cake. Place sliced bananas on the topping, drizzle the remaining ½ cup of melted peanut butter on top, and sprinkle with the roasted peanuts. Chill the cake for at least 3 hours or up to overnight.