In a large bowl, use a hand mixer fitted with a whisk attachment to beat the sweetened condensed milk and cold water together for about 1 minute, until fully combined.

Sprinkle the entire (dry) instant vanilla pudding mix into the condensed milk mixture and beat until completely smooth, about 2 minutes. Transfer this pudding mixture to a separate bowl, cover, and refrigerate for 30–60 minutes, or until firm.

Fit a stand mixer with the whisk attachment. Add the heavy cream, the vanilla paste or extract (if using), and the salt to the mixer bowl.

Beat the cream on medium speed for about 1–3 minutes, until it begins to thicken. Increase the speed to medium-high and whip until soft peaks form. Stop as soon as soft peaks appear to avoid overwhipping.

Reduce the mixer to low. While it runs, add the chilled pudding mixture one large spoonful at a time, continuing to mix until no streaks remain and the combined mixture is smooth and nicely thickened.

Reserve 4 vanilla wafers for garnish. Slice the 6 bananas.

To assemble: spread 1/4 of the pudding mixture evenly over the bottom of a trifle bowl or serving dish. Arrange a layer of vanilla wafers over the pudding, then add a layer of sliced bananas. Spread a second 1/4 of the pudding over the bananas.

Repeat the layers (cookies, bananas, pudding) until the pudding, cookies, and bananas are used up, ending with the remaining 1/4 of pudding as the top layer.

Cover the assembled dessert tightly with plastic wrap and refrigerate for 3–6 hours, or until the cookies have softened to a cake-like consistency.

Just before serving, crush the reserved 4 vanilla wafers and sprinkle them over the top. Serve the dessert within 12 hours of assembling.