Put oven rack in middle position and preheat oven to 425°F.
Pour condensed milk into pie plate and stir in a generous pinch of salt. Cover pie plate with foil and crimp foil tightly around rim. Put in a roasting pan, then add enough boiling-hot water to reach halfway up side of pie plate, making sure that foil is above water.
Bake, refilling pan to halfway with water about every 40 minutes, until milk is thick and a deep golden caramel color, about 2 hours.
Remove pie plate from water bath and transfer toffee to a bowl, then chill toffee, uncovered, until it is cold, about 1 hour.
While toffee is chilling, clean pie plate and bake piecrust in it according to package instructions. Cool piecrust completely in pan on a rack, about 20 minutes.
Spread toffee evenly in crust, and chill, uncovered, 15 minutes.
Cut bananas into 1/4-inch-thick slices and pile over toffee.
Beat cream with brown sugar in a clean bowl with an electric mixer until it just holds soft peaks, then mound over top of pie.
• Toffee can be chilled up to 2 days (cover after 1 hour).• Toffee-filled crust can be chilled up to 3 hours.
