Step 1

Whisk 2 Tbsp. sugar, 1 Tbsp. cornstarch, and ¼ tsp. kosher salt in a medium saucepan to combine, then whisk in 1 large egg and ¾ cup whole milk. Set pan over medium-high heat and bring mixture to a boil, whisking constantly, about 5 minutes. Reduce heat to medium and continue to cook, whisking, 1 minute more. Remove from heat and whisk in 1 Tbsp. unsalted butter and ¾ tsp. vanilla extract. Scrape pudding into a large bowl with a rubber spatula and chill while you prep the banana and condensed milk mixture (no need to cover).

Step 2

Peel 1 banana and chop into ½" pieces. Whisk ¾ cup sweetened condensed milk and ½ cup mascarpone or room-temperature cream cheese (if using) in a small bowl until combined and smooth.

Step 3

Fold banana and sweetened condensed milk mixture into warm pudding with rubber spatula (adding the banana to the warm pudding helps flavor it). Return pudding to the refrigerator and chill 1 hour (no need to cover). It’s okay if the pudding is still quite loose at this stage. (If you aren’t using the mascarpone, add the banana and sweetened condensed milk when you transfer the warm pudding from the saucepan to the large bowl.)

Step 4

Using an electric mixer, beat 1½ cups heavy cream in another small bowl until stiff-peaks form, about 4 minutes. (Err on the side of too stiff rather than too loose.) Whisk chilled pudding if lumpy and gently fold two thirds of whipped cream into pudding; set remaining whipped cream aside for serving.

Step 5

Slice remaining 4 large or 5 medium bananas into ½"-thick rounds. Set aside a large handful of cookies from one 11-oz. box Nilla Wafers and set the rest next to a 2-qt. or 8x8" baking dish or pan.

Step 6

Evenly spread about one third of pudding across bottom of dish in an even layer. Top with a layer of cookies, spacing evenly apart and not covering the pudding completely. Top with half of the bananas. Repeat layers once more, using half of the remaining pudding. Then top with a final layer of pudding and reserved whipped cream. (You will not use the entire box of cookies—the extras are the chef’s treat!)

Step 7

Place reserved cookies in a resealable plastic bag, close, and finely crush with a rolling pin. (A mortar and pestle also works well.) Sprinkle crushed cookies over pudding. Cover with plastic wrap or a silicone lid and chill at least 6 hours before serving.Do ahead: Pudding can be assembled 3 days ahead. Keep chilled.