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, 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 ½-inch plain tip with the lemon curd.
Fill the tart shells with curd to ¼ inch from the rim of the shells (50 grams/¼ cup each).
Place the cake rounds on top.
Hold the pastry bag vertically over the center of each cake, about ½ 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.