Method

• 1.
  In a heavy-bottomed medium saucepan, heat 2 tablespoons of the butter over medium heat. (Refrigerate the remaining butter for whisking into the finished pudding.) When the butter is foaming, add the banana and cook, occasionally stirring and mashing the pieces with the back of a heatproof flexible spatula or wooden spoon, until the banana is browned in spots and very soft and mushy, 5 to 7 minutes. Remove the saucepan from the heat.
• 2.
  Add the milk, dulce de leche, and salt to the saucepan and blend with a handheld blender on high until the mixture is completely smooth. (Alternatively, scrape the banana mixture into a standard blender, add the milk, dulce de leche, and salt and blend until smooth, then return to the saucepan.) Heat the milk mixture over medium heat, whisking occasionally, until it’s steaming and just starting to ripple beneath the surface, about 5 minutes. Remove the saucepan from the heat.
• 3.
  In a medium bowl, combine the cornstarch and sugar and whisk until combined and free of lumps. Add the yolks and whole egg and whisk to combine, then whisk vigorously until the mixture is slightly pale, thickened, and light, about 2 minutes. Whisking the egg mixture constantly, slowly pour about two-thirds of the hot milk mixture into the bowl, to temper the egg mixture, then whisk the warmed contents of the bowl back into the saucepan with the remaining milk mixture.
• 4.
  Have a clean medium bowl nearby for transferring the hot pudding. Set the saucepan back over medium heat and cook the mixture, whisking constantly and scraping around the sides and bottom of the saucepan, until the pudding is thickened, the foam on the surface has subsided, and it holds the marks of the whisk, about 4 minutes. Stop whisking for a few seconds and check for slow bubbling beneath the surface, indicating the mixture is at a boil, then continue to whisk vigorously for another 15 seconds. Immediately remove the saucepan from the heat and pour the pudding into the reserved medium bowl.
• 5.
  Cut the reserved 4 tablespoons (2 oz / 57g) chilled butter into 1/2-inch pieces, then whisk the butter into the pudding a few pieces at a time, waiting for the pieces to disappear before adding more, until all the butter is incorporated and the mixture is smooth. Whisk in the vanilla. Press a piece of plastic wrap directly onto the surface of the pudding and refrigerate until it’s cold and set, at least 4 hours.
• 6.
  In a large bowl, with a hand mixer, whip the heavy cream and sour cream on low speed to start and gradually increase the speed to medium-high as the mixture thickens, until you have a firmly whipped cream that holds a stiff peak.
• 7.
  Remove the pudding from the refrigerator and stir to loosen the consistency. Spread about one-quarter of the pudding in an even layer in the bottom of the 2-quart glass dish or serving bowl, then spread about one-quarter of the whipped sour cream over the top. Top with one-third of the digestive biscuits, breaking them into pieces as needed to fit in an even layer, and drizzle with 1 tablespoon of the dulce de leche. Repeat the layering process two more times with the same quantities of pudding, cream, biscuits, and dulce de leche (you’ll use all the biscuits but have more of the pudding, whipped sour cream, and dulce de leche). Spread the remaining pudding over the final layer of biscuits, then scrape the remaining cream mixture on top and smooth almost to the edges. Drizzle the remaining 1 tablespoon of dulce de leche over the top, then use the back of a spoon to swirl the dulce de leche into the cream. Loosely cover the dish and refrigerate for at least 4 hours to allow the biscuits to soften before scooping into bowls and serving.