Make the vanilla custard: Meanwhile, in a medium saucepan, bring the milk and vanilla to a simmer. In a medium bowl, whisk together the egg yolks, sugar, cornstarch and salt until thick and smooth. Whisking constantly, add about ¼ cup of the hot milk mixture to temper the eggs. Gradually whisk in the remaining milk until combined; return to the saucepan and cook, whisking constantly, over low heat until thickened, about 5 minutes. Remove from the heat and whisk in the butter until smooth. Let cool slightly, stirring occasionally. Using a sieve, strain the cream, if necessary, into a medium bowl and cover the surface directly with plastic wrap; refrigerate until cold, at least 2 hours.
Make the whipped cream topping: Beat the cold whipping cream with the confectioners’ sugar, sour cream and vanilla until soft peaks form.
To assemble the pie, spread the dulce de leche evenly over the cooled pie shell. Top with 2 layers of bananas. Beat the cold custard with a whisk until smooth; spread evenly over the bananas. Refrigerate overnight, covered, or to serve, top decoratively with the whipped cream topping, sprinkle with cookie crumbs and top with the banana slice.