Make crust: Preheat oven to 350ºF. Mist a 9-inch pie plate with cooking spray. Pulse Nilla wafers and sugar in a food processor until ground and combined. Add butter; pulse until crumbly and evenly moistened. Press into pie plate. Bake until edges are golden, about 10 minutes. Let cool completely on a wire rack.
Make filling: Combine milk, cream and 1/2 cup sugar in a heavy-bottom saucepan. Set over medium-high heat and bring just to a gentle boil, stirring to dissolve sugar. In a medium bowl, whisk cornstarch, salt and remaining 2 Tbsp. sugar; add egg yolks and whisk until smooth. When milk mixture just begins to boil, remove from heat and slowly drizzle about 1 cup into egg yolk mixture, whisking vigorously. Stir that back into pan and return to stove. Lower heat to medium and cook, stirring constantly, until custard thickens and comes to a boil. Boil gently for 1 minute, then remove from heat and stir in butter and vanilla until melted and smooth.
Cut 2 bananas into 1/4-inch-thick slices and spread over bottom of crust. Mash remaining banana with a fork and stir into custard. Spoon custard into crust and smooth top. Cover with plastic wrap, gently pressing wrap directly onto custard surface to keep a skin from forming. Refrigerate until completely cool and custard has firmed, at least 3 hours.
Just before serving, whip cream and confectioners' sugar until soft peaks form. Remove plastic wrap and spread whipped cream over top of pie.