Cost per Serving:
Pour condensed milk into a large, heatproof bowl and set over a pot of simmering water to make dulce de leche. Cook, checking water level and adding water as necessary. Stir milk occasionally and cook until it is very thick and golden brown, about 2 1/2 hours.
Bake crust according to package directions. Cool on a wire rack.
Slice 2 bananas. Spread half of warm dulce de leche in crust; top with sliced bananas. Spread remaining dulce de leche over bananas. Chill until cold, at least 2 hours.
Just before serving, combine heavy cream and sour cream in a bowl. Whip, using a mixer on high speed, until soft peaks form. Spread over pie. Slice remaining banana and use to garnish pie. Cut pie into wedges and serve.