Place a rack in the center of the oven and preheat the oven to 400°F.
Mist a 9" pie pan with cooking spray. Set the pie pan aside.
Place the bananas in a large mixing bowl and beat them with an electric mixer on low speed until smooth. You should have 1 cup of banana puree. Stop the machine and add the Bisquick, eggs, milk, butter and vanilla. Beat on low speed until the ingredients are well blended, 1-1 1/2 minutes longer. Using a rubber spatula, scrape the batter into the prepared pie pan and smooth the top.
Bake the pie until it is well caramelized on top and firm around the edge but still a little soft in the center, 25-30 minutes. Remove the pie from the oven and let it rest 30 minutes before slicing. Serve warm with vanilla ice cream, or chilled with sweetened whipped cream.