Preparation

Make the crust

• Arrange eight 3-inch-diameter, 2-inch-deep round metal ring molds on a rimmed baking sheet lined with parchment.

  In a large bowl, stir the cracker crumbs, melted butter, and sugar until the crumbs are evenly moist and slightly clump together. Divide the mixture among the molds and pack it in, pressing to compact. Refrigerate.

Make the lemon layer

• Fill a large bowl about a third full with ice cubes plus a cup or so of water. Set a second bowl (one that holds at least 2 quarts) in the ice bath and put a medium-mesh strainer in the bowl.

  Combine the sugar and lemon zest in a medium bowl and rub it between your fingers to release the lemon oil into the sugar.

  In a large heatproof bowl, whisk the lemon juice, eggs, egg yolks, and the sugar mixture. hisk in the butter pieces. In a 4-quart saucepan, bring 2 inches of water to a bare simmer. Set the bowl over the pot (the water shouldn’t touch the bottom of the bowl). Whisk constantly until the mixture thickens and reaches a temperature of 170°F, 5 to 10 minutes. Immediately strain into the bowl in the ice bath. Gently stir every so often until completely cool.

  Distribute 1/2 cup of the lemon curd evenly among the 8 ring molds and spread it with the back of a spoon to cover the crust. Put the baking sheet in the freezer.

  In a stand mixer fitted with the whisk attachment, whip the cream just until soft peaks form, 1 to 2 minutes. With a spatula, gently fold the remaining lemon curd into the whipped cream until well combined. Spoon about 1/2 cup of lemon cream into each mold (you may not use all of the cream). Cover loosely with plastic and freeze for at least 8 hours or overnight.

Make the caramel sauce

• In a 4-quart heavy-duty saucepan, mix the sugar, salt, and 1/2 cup water with a wooden spoon. Cook over medium-low heat until the sugar turns golden-amber, about 15 minutes. As the sugar cooks, occasionally rinse down the sides of the pan with a wet pastry brush to prevent sugar crystals from forming. Remove the pan from the heat and carefully add the cream—it may splatter a bit. Stir in the vanilla. If the sugar hardened when adding the cream, stir until it’s completely dissolved. Let cool to room temperature, about 4 hours.

Make the meringue

• Shortly before serving, boil the sugar and 1/2 cup water in a heavy-duty 2-quart saucepan over medium heat, stirring to dissolve the sugar, until the mixture reaches the soft-ball stage (235°F to 240°F on a candy thermometer), 3 to 5 minutes. As the sugar boils, occasionally wash down the sides of the pan with a wet pastry brush to prevent sugar crystals from forming.

  Meanwhile, whip the egg whites and salt in a stand mixer fitted with the whisk attachment on low speed until foamy. Increase the speed to medium, add the cream of tartar and beat until soft peaks form, about 3 minutes. With the mixer on medium speed, pour the hot sugar syrup in a very thin stream down the side of the bowl. Continue beating on medium-high speed until the egg whites are firm and glossy and the bowl is cool to the touch, 6 to 8 minutes.

Assemble the cakes

• Put the cakes on individual serving plates. To unmold, quickly pass the flame of a small kitchen torch around the sides of the rings to loosen them, and then use tongs to slide the rings off. Dollop spoonfuls of meringue over the lemon cream. Using the kitchen torch, lightly brown the meringue. Pool about 2 Tbs. of the caramel sauce next to each cake and garnish with blueberries. Serve immediately.

Make Ahead Tips

The sauce may be made several days ahead. Refrigerate and gently reheat just long enough to take off the chill before serving. Leftovers will keep, refrigerated, for about 2 weeks.