Blind bake a deep pie crust or one that is large enough to hold 3 cups of filling. Set aside to cool.
In a large bowl, whisk sugar, flour and salt together. Slowly add in the milk to the flour and sugar to incorporate and prevent lumps. Transfer this carefully to a pot. Heat milk mixture over medium to medium-low heat for about 7 minutes, whisking constantly to ensure as the filling is heating up, it is not coming to a rolling boil. As it heats, after about 5 minutes, you will notice that it will start to thicken. Once it starts to thicken, cook an additional 2 minutes and remove from heat. Helpful hint: if mixture starts to bubble consistently once it has started to thicken, reduce the heat. If you were to NOT stir, you should see the occasional bubble. If you are continuously stirring, you shouldn't really see any bubbles. (If your whisk doesn't reach into the corners of the pot, switch every so often to a wooden spoon or rubber scraper to ensure the bottom isn't scorching around the edge of the pot.)
Whisk egg yolks in a separate bowl. Temper the egg yolks by whisking in 1/2 cup of the hot, thickened milk mixture. Then whisk the tempered egg yolks back into the pot. Replace back onto the heat and cook on medium to medium-low for 2-3 minutes more. It will continue to thicken. Remove from heat.
Stir in butter and vanilla. Pour about 1/3rd of the filling to the bottom of the pie crust. Add sliced bananas and cover with remaining filling. Cover the top directly with plastic wrap and chill 4-6 hours or until set, completely chilled and plastic wrap removes cleanly. Overnight is fine!
To serve, whip cream with powdered sugar and vanilla. Top pie with cream and any extra sliced bananas, if desired. Cut into 8 pieces and serve.