Make the cakes: Preheat the oven to 350 degrees F. Butter and flour a 6-cup jumbo muffin pan. Beat the egg yolks, 1/3 cup sugar, the vanilla and lemon zest in a large bowl with a mixer on medium-high speed until pale, fluffy and doubled in volume, 3 to 6 minutes. Meanwhile, bring a medium saucepan with a few inches of water to a simmer, then turn off the heat. Combine the egg whites and salt in a large heatproof bowl and place on top of the saucepan (do not let the bowl touch the water). With clean dry beaters, beat the egg whites on medium speed until soft peaks form, about 1 minute, then increase the speed to medium high and gradually add the remaining 1/3 cup sugar; continue beating until stiff peaks form, 2 to 3 more minutes. If the mixture gets too hot, remove the bowl from the pan.
Gently fold the egg whites into the yolk mixture with a rubber spatula in four batches, sifting in the flour in alternating batches. It's OK if some streaks remain -- do not overmix. Remove 1/2 cup of the batter to a small bowl and whisk in the melted butter, then gently fold back into the batter. Divide the batter among the muffin cups. Bake until the centers spring back when pressed, 18 to 20 minutes. Let cool 5 minutes in the pan on a rack, then loosen with a paring knife and remove to the rack to cool completely.
Meanwhile, make the filling: Place a fine-mesh sieve in a medium bowl and set in a larger bowl of ice water. Whisk 1/4 cup milk, 2 tablespoons sugar, the egg yolks and cornstarch in a separate medium bowl until smooth. Heat 1 cup milk, 2 tablespoons sugar and the salt in a small saucepan over medium heat until steaming, whisking to dissolve the sugar. Whisking constantly, gradually pour the hot milk into the egg yolk mixture. Return the mixture to the saucepan and cook, whisking frequently, until thickened, 2 to 3 minutes, then continue whisking for 30 more seconds. Remove from the heat and stir in the butter and vanilla. Immediately strain into the bowl set in the ice water and let cool, stirring occasionally, until lukewarm. Remove from the ice water, press a piece of plastic wrap directly on the surface of the filling and refrigerate until completely cooled. When ready to assemble the cakes, whisk in the lemon curd, limoncello and lemon juice. Whip the heavy cream in a medium bowl until stiff peaks form and fold into the filling.
Make the lemon syrup: Combine the water, sugar and lemon zest in a small saucepan. Bring to a boil, stirring to dissolve the sugar, then remove from the heat. Stir in the limoncello. Let the syrup cool.
Fill the cakes: Trim the domed tops of the cakes to make them level (this will be the bottom). With a paring knife, cut a 1 1/2-inch-wide hole in the cakes; scoop out some of the inside to make a cavity, reserving the scraps. Brush the insides with some of the lemon syrup, then fill with the lemon filling, reserving the rest of the filling for the glaze. Close the hole with the cake scraps. Brush the cakes all over with the syrup until all the syrup is used.
Glaze the cakes: Place the cakes trimmed-side down on a rack set on a baking sheet. Whisk 5 to 6 tablespoons milk into the reserved lemon filling until pourable. Spoon over each cake to completely cover. Transfer to a platter and refrigerate, uncovered, at least 4 hours or overnight.
Garnish the cakes: Beat the heavy cream and sugar in a medium bowl until stiff peaks form. Transfer the whipped cream to a piping bag fitted with a star tip and pipe around and on top of each cake. Top with candied lemon peel.