Bake your pie shell according to package directions and set aside to cool.
In a medium size bowl, whisk together the sugar, salt, cornstarch, and ½ cup of the cold milk until smooth. Add the egg yolks and whisk until mixed.
In a large (3 quart) saucepan, heat remaining 2 cups of the milk until hot and bubbly.
Gradually whisk the hot milk into the egg mixture, then pour it all back into the saucepan and whisk over medium heat until it thickens (big bubbles will start to break the surface and it will be thick like pudding). Add the marshmallows and whisk just until they melt.
Remove from heat and whisk in the butter and vanilla.
Pour the saucepan mixture into a bowl, press a piece of greased wax or parchment paper over to keep skin from forming and let cool to room temperature.
In a medium bowl, toss the bananas with the pineapple juice until evenly coated. Drain and discard any juice. Stir the bananas into the almost-cool pie filling, and pour into the baked crust. Press plastic wrap directly onto the filling and chill for at least 4 hours or until ready to serve.
Before serving, whip the cream until soft peaks form, then add powdered sugar. Start with ¼ cup and add more to taste (up to ⅓ cup) and beat until stiff peaks form. Beat in vanilla. Pipe or spread over the pie.
Notes
I recommend baking the pie shell earlier or putting it in the oven as soon as you start making the cream filling so that it will have enough time to cool.