1. In a bowl, whisk the egg yolks with the cornstarch, the 1/2 cup of sugar and 1/4 teaspoon of salt. In a medium saucepan, bring the milk to a boil. Gradually whisk the milk into the egg yolks until smooth. Transfer the pudding mixture to the saucepan and add the banana liqueur. Cook over moderate heat, whisking, until the pudding is thick, about 3 minutes. Scrape the pudding into a bowl and whisk in the cold butter and vanilla. Cover with plastic and refrigerate until chilled, about 4 hours.
2. Preheat the oven to 325°. Line a baking sheet with parchment paper. In a bowl, combine the wafers, cinnamon, the remaining 2 teaspoons of sugar and a pinch of salt. Stir in the melted butter. Spread the crumble on the prepared baking sheet; bake for 15 minutes, until lightly browned. Let cool.
3. Spoon the chopped bananas into bowls. Top with the pudding, sprinkle with the crumble and serve right away.