Meanwhile, whisk the eggs and sugar together in a medium saucepan. Slowly whisk in the lemon or lime juice. Place the pan over medium heat and whisk slowly, until the mixture begins to simmer. Simmer for 3 to 5 minutes, whisking constantly, until thickened. Remove the pan from the heat and whisk gently for 1 to 2 minutes to release steam and cool the curd slightly.
Remove the gelatin from the water, squeezing out the excess water, and whisk it into the hot curd. Strain the curd through a fine-mesh strainer set over the container of a Vitamix and blend on low speed for a few seconds, then add the butter 2 or 3 pieces at a time, blending until incorporated. Add the zest, if using, and blend to incorporate. Let the curd cool to room temperature.
The curd can be used at this point or transferred to a covered container. Press a piece of plastic wrap against the surface to prevent a skin from forming and refrigerate for up to 4 days.
If the curd has been refrigerated and has stiffened, transfer it to the bowl of a stand mixer fitted with the paddle attachment and mix slowly until it reaches a creamy consistency.
To Assemble: For the tart shell: Preheat the oven to 325°F. Line a sheet pan with a Silpat or parchment paper.
Roll out half the dough, line the tart rings, and fill with rice. Bake for 8 minutes, then rotate the pan and bake for another 8 minutes, or until the dough is set and no longer sticks to the parchment paper. Remove the parchment and rice (store the rice for future use), return the pan to the oven, and bake for another 12 minutes, or until the dough is cooked through and golden brown. Set the pan on a cooling rack and cool completely. Run the tip of a paring knife between the top of the crust and the ring to loosen the crust, then lift off the ring.
Meanwhile, make a Swiss meringue: Place the egg whites and sugar in the bowl of a stand mixer and set over a saucepan of simmering water. Whisk until the mixture reaches 160°F/71.1°C, then immediately put the bowl on the mixer, attach the whisk attachment, and whip at medium-high speed for about 5 minutes, until the whites hold stiff peaks and are cool. Transfer to the pastry bag fitted with the 1-inch tip.
Use the round cutter to cut the frozen madeleine layer into 6 rounds.
Fill the pastry bag fitted with the 1/2-inch plain tip with the lemon curd.
Fill the tart shells with curd to 1/2 inch from the rim of the shells (50 grams/1/3 cup each). Place the cake rounds on top. Hold the pastry bag vertically over the center of each cake, about 1 inch from the surface, and pipe the meringue to cover about two-thirds of the surface (20 grams each), then pull the bag up to form a dome with as much additional meringue as you’d like.
Hold a small offset spatula at the tip of the domed meringue and pull it to the edges of the crust, forming a smooth line from the tip to the outside of the shell. Wipe off the spatula and continue around the tart to form a cone shape, then use the spatula to form irregular peaks in the meringue. Using the propane torch, brown the top of the meringue.
The tarts are best the same day they are assembled. They can be refrigerated for 4 to 6 hours; remove them from the refrigerator 20 minutes before serving.