4 Tbsp. unsalted butter, melted, slightly cooled, plus more for pan
2 cups finely crushed Nilla wafers
3 Tbsp. granulated sugar
½ tsp. kosher salt
Filling and assembly
3 (8-oz.) packages cream cheese, room temperature
3 ripe bananas
1 (14-oz.) can sweetened condensed milk
4 large eggs, room temperature
1 tsp. kosher salt
1 Tbsp. vanilla bean paste or 1 vanilla bean, split lengthwise
2 cups heavy cream
½ cup full-fat sour cream
¼ cup powdered sugar
Crust
Place a rack in middle of oven; preheat to 325°F. Line a 13x9" or 12x8" baking pan with two pieces of parchment paper, leaving overhang on all sides. Anchor parchment to pan with metal binder clips. Stir Nilla wafers, sugar, salt, and remaining 4 Tbsp. butter in a medium bowl to combine. Press mixture into an even layer in bottom of prepared pan; it won’t hold together but will pack like a sand. Bake until toasted, about 10 minutes. Let cool.
Filling and assembly
While the crust is cooling, beat cream cheese in the bowl of a stand mixer fitted with the paddle attachment on medium-high speed, stopping motor occasionally and scraping down sides of bowl and beater, 3 minutes. Reduce speed to medium and drop in pieces of banana, a little at a time, breaking up and incorporating completely before adding more. (Adding the bananas slowly helps blend them into the cream cheese.) Scrape down bowl and beater again, then beat 2 minutes on medium-high, covering mixer with a splash guard or kitchen towel to avoid spatter. Reduce speed to medium-low and pour in condensed milk; beat, scraping down bowl halfway through, 2 minutes. Reduce speed to low and add eggs, one at a time, mixing well after each addition; mix in salt. For the silkiest cheesecake, strain filling though a fine-mesh sieve into a jug or pitcher, pushing through with a rubber spatula. Stir in vanilla paste or scrape in vanilla seeds and mix well.
Knock stand mixer bowl or jug against counter a few times to release any trapped air. Let filling sit 2 minutes, then pour into crust. Bake cheesecake until 2" of edges are puffed and center wobbles slightly when pan is gently shaken, 30–35 minutes, depending on size of pan. Turn off oven and prop open door (a wooden spoon works well). Let cheesecake cool in oven 30 minutes. Transfer pan to a wire rack and let cheesecake cool completely. Chill until cold, at least 4 hours, covering with wax paper or plastic wrap after 1 hour.
When you are ready to top cheesecake, beat heavy cream, sour cream, and powdered sugar in the clean bowl of a stand mixer with whisk attachment on medium-high until stiff peaks form, about 5 minutes. Spread topping over chilled cheesecake. Chill, uncovered, at least 1 hour before serving.
Slice cheesecake with a hot, dry knife and serve cold or room temperature.