Crush the graham crackers in a food processor, then add the melted butter and salt, and pulse until it resembles wet sand and clumps together when you squeeze it. If you don't have a food processor, you can put the crackers in a Ziploc bag and use a rolling pin to crush them into crumbs.
Tip the mixture into your pie dish and press it into the bottom and sides of the pan.
Optional. If you want to make the crust extra sturdy, bake it at 350°F / 180°C for 10 minutes to help it solidify. This is not necessary, but it will help the pie to hold together when you slice it later.
Make the Banoffee Filling
Chop bananas into slices about ¼ inch thick. Reserve enough banana slices to cover the top of the pie in a layer of banana, then chop the remaining slices into quarters to make banana chunks.
Melt the butter and brown sugar in a saucepan over medium heat, stirring slowly but constantly until the sugar has dissolved. (About 1-2 minutes).
Add the condensed milk and salt and bring to a boil. (About 4 minutes.Once you see very thick bubbles bursting on the surface, boil for a further 1-2 minutes, then remove from the heat. It is important not to undercook the caramel, or it won't set.
Immediately put the quartered banana chunks into the hot caramel and stir to combine. The heat will cause the banana flavor to infuse into the caramel filling.
Pour the banoffee filling into the pie dish and lay the reserved banana slices on top. Cover with plastic wrap and let the pie cool to room temperature before transferring to the fridge for 20 minutes.
Decorate
Once the caramel is cool to the touch, whip the cream and powdered sugar up to stiff peaks, and pipe or spread it all over the pie.
Chill for at least two hours before serving.
If you like, you can decorate the pie with caramel sauce, chocolate shavings, cocoa powder, or whatever you want.
Notes
You don't have to have the layer of bananas on top if you don't want to. I often just quarter all the bananas and mix them into the caramel filling. I actually prefer it this way. But if you want to see a distinctive layer of bananas between the caramel and cream, then you can. Note: Do not put the cream on top of the pie until the caramel is cool, or the cream will melt!