2 tablespoons confectioners’ sugar, for dusting FOR THE ASSEMBLY:
1 cup fresh or thawed frozen blueberries, strawberries, blackberries or raspberries
Instructions
Make the topping: Beat cream and confectioners’ sugar with an electric mixer until it just holds stiff peaks.
Make the crumbs: Preheat oven to 350˚F with rack in middle. Coarsely crush vanilla wafers between sheets of plastic wrap with a rolling pin or bottom of a heavy skillet, then stir together with butter and salt in a bowl. Spread mixture on a baking sheet and toast in oven until lightly browned, about 5 minutes. Cool on sheet, then break up with a fork.
Make the custard: Stir together yolks in a small bowl. Whisk together sugar, cornstarch and salt in a small heavy saucepan. Slowly whisk in water and milk, then whisk until smooth. Bring to a boil over medium heat, whisking frequently as mixture begins to thicken. Remove from heat and gradually whisk about 1/2 cup milk mixture into yolks, then whisk yolk mixture into pan. Add lemon zest and juice and simmer, whisking constantly, 3 minutes. Remove from heat and whisk in butter until incorporated. Transfer custard to a metal bowl, then set bowl in a larger bowl of ice water and stir custard until cooled to room temperature. Cover surface with wax paper to prevent a skin from forming.
Assemble parfaits: Reserve 2 teaspoons crumbs and 4 raspberries for garnishing all parfaits. Assemble one parfait in layers using a total of 1/4 cup plus 1 tablespoon custard, 1/4 cup berries, a rounded 1/4 cup crumbs and one fourth of whipped cream. Assemble 3 more parfaits, then top with reserved crumbs and berries. Serve parfaits at room temperature or chilled.
Make ahead: Assembled parfaits may be made 3 hours ahead and kept refrigerated, uncovered.