To prepare crust: Preheat oven to 350 degrees F. Coat a 9-inch tart pan (with removable bottom) with cooking spray.

Blend graham cracker crumbs and butter in a medium bowl with your fingers until well combined. Press the mixture into the bottom and halfway up the sides of the prepared pan. Bake until lightly browned, about 10 minutes. Cool on a wire rack.

To prepare filling: Increase oven temperature to 425 degrees . Put a kettle of water on to boil.

Pour condensed milk into a glass or ceramic pie pan. Stir in salt. Tightly cover the pan with foil and place in a large roasting pan. Carefully pour 1 inch of boiling water into the roasting pan and place in the oven. Bake until the condensed milk is thickened, sticky and caramel in color, about 1 3/4 hours. Check about halfway through and add more boiling water if the level is low.

Reserve 2 tablespoons of the condensed-milk toffee in a small bowl. While it's still warm, dollop the remaining toffee onto the crust (it will be sticky and a little lumpy); gently spread to cover the bottom. Cover and refrigerate the reserved toffee and the pie until chilled, about 1 hour. (If making ahead, after 1 hour loosely cover and refrigerate for up to 1 day.)

Remove the pie from the refrigerator and arrange 3 of the sliced bananas over the toffee.

Beat cream in a bowl with an electric mixer on medium speed until soft peaks form. Add vanilla; beat until stiff. Spread the whipped cream over the pie, leaving a 1-inch border around the edge. Garnish with the remaining banana. Stir 1 teaspoon water into the reserved toffee until smooth and drizzle over the pie. Remove the pan sides before cutting. Serve immediately.