Put the sugar and salt in a medium saucepan. Sift in the cornstarch, and whisk the mixture to reduce lumps. Add 2 of the eggs plus 1 egg yolk (reserve the white), and whisk vigorously until the mixture is well combined and light yellow, about 2 minutes. Add the milk and whisk to combine.

Put the saucepan over medium-low heat, and cook, stirring gently and constantly with a wooden spoon, until the mixture begins to thicken, 15 to 18 minutes. (This will happen at around 175°F; use a candy thermometer if you have one.) Once the pudding has thickened, immediately remove the saucepan from the heat and whisk in the cold butter, one slice at a time, until the mixture is glossy and smooth. Add the vanilla extract and banana liqueur and whisk to combine.

Transfer the pudding to a medium bowl and cool it to room temperature, then cover it with plastic wrap and refrigerate until chilled through, about 2 hours. Once the pudding has chilled, fold in the banana slices.

Whip 1 cup of the heavy cream with the whisk attachment of a stand mixer (or a handheld electric mixer) until frothy. Add 1 tablespoon of the caster sugar, and continue whipping until the cream is fluffy and forms soft peaks. Using a rubber spatula, gently fold the whipped cream into the pudding until just combined. Refrigerate the pudding while you make the vanilla-bean mousse.

Whip the remaining 1 cup heavy cream with the whisk attachment of a stand mixer (or a handheld electric mixer) until frothy. Add ¼ cup of the caster sugar and the vanilla-bean seeds, and continue whipping until the cream is fluffy and forms soft peaks. In another medium bowl, whisk 2 egg whites until foamy, add the remaining ¼ cup caster sugar, and continue beating until stiff peaks form. (Discard the remaining egg yolk, or save it for another use.) Using a rubber spatula, gently fold the egg whites into the whipped cream just until combined, trying to preserve as much volume as possible. (You can refrigerate the mousse for up to 2 hours before assembling the final dish.)

Put about ⅔ of the pudding in a large glass dish. Top with an even layer of 25 to 30 Nilla Wafers, then add the remaining pudding. Spread the mousse evenly over the pudding. Roughly crumble the remaining Nilla Wafers over the mousse. Cover loosely with plastic wrap, refrigerate for at least 1 hour, and serve. The pudding’s texture will be best if served on the day of preparation, but refrigerating overnight is fine. (Due to the raw eggs in the mousse, do not keep leftovers longer than two days.)