Prepare the frozen pie crust as instructed on the package.
Set the oven to 350F.
Combine the milk, cream, eggs, sugar, salt, cornstarch, and gelatin in a large saucepan. Stir until well blended and heat over medium-low heat while whisking often. When the custard has thickened and becomes difficult to stir, (approximately 10-15 minutes) turn the heat off and stir in the butter and vanilla. Set aside.
Peel and slice the bananas and line the bottom of the piecrust with two layers of sliced bananas.
Pour the custard over the bananas and smooth the custard out throughout the pie.
Bake for 20-25 minutes.
Remove pie from oven and cool completely before transferring to the refrigerator. Chill for a few hours or overnight before serving.
To serve, top with whipped cream, additional banana slices, and/or a cherry on top.