In a small saucepan, combine sugar, cornstarch, salt, and cream. Whisk over medium heat until hot and bubbly*. Let boil for 1 minute and remove from heat.
Whisk ½ cup of the hot mixture into the egg yolks a spoonful at a time until incorporated**.
Pour the egg mixture back into the pot and whisk. Heat on medium heat, and bring to a boil. Reduce heat to simmer and let cook 1 minute. Remove from heat and stir in vanilla and butter.
Add sliced bananas to the pie crust. Pour warm custard over the bananas.
Refrigerate at least 4 hours or overnight. Once cooled and set, top with whipped cream.
Notes
*Heating at too low of a temperature may cause the mixture to not set as well.**Tempering eggs just means to warm them slowly so they don’t turn into scrambled eggs or get lumpy when mixed into a hot liquid. To temper eggs, whisk eggs in a bowl. Add a little bit of hot liquid (usually about 1/2-1 cup) while whisking the eggs. This will gradually bring the temperature up. Next, add the egg mixture back into the remaining hot mixture and cook until thickened.