Put the lemon zest in a 4-quart or larger bowl and set a medium-mesh sieve on top. In a heavy-duty 4-quart saucepan, whisk the egg yolks and sugar. Add the lemon juice, butter, and salt. Cook over medium-low heat, stirring constantly, until thickened enough to coat the back of a wooden spoon but still pourable, 4 to 5 minutes. (Don’t boil or it will curdle.) Pass the thickened curd through the sieve and mix in the zest. Cool, stirring occasionally, about 1 hour.
When the lemon curd is cool, beat the cream with an electric mixer on medium speed just until soft peaks form, about 2 minutes. With a large balloon whisk or silicone spatula, fold in the lemon curd. Cover the bowl tightly with plastic wrap and refrigerate.
Make the meringue
In a small, microwaveable bowl, sprinkle the gelatin over 3 Tbs. water; let soften for at least 5 minutes. Microwave on high to melt the gelatin, 15 to 30 seconds.
In a heavy-duty nonstick 2-quart saucepan over medium-high heat, combine 1 cup of the sugar and 6 Tbs. water and stir constantly until the syrup is bubbling, 2 to 3 minutes. Remove the pan from the heat.
In a stand mixer fitted with the whisk attachment, beat the egg whites on medium-high speed until foamy, 45 seconds. Add the cream of tartar and beat until soft peaks form, 30 seconds. Gradually beat in the remaining 3 Tbs. sugar until stiff peaks form, 1 to 2 minutes.
Have ready a 2-cup or larger heatproof liquid measure. Return the pan of syrup to medium-high heat and boil until a candy thermometer registers 248°F (firm ball stage). Pour the syrup into the measure to stop the cooking and then immediately pour a small amount of syrup over the whites with the mixer off. Immediately beat at high speed for 5 seconds. Stop the mixer and add a larger amount of syrup. Beat at high speed for 5 seconds. Continue with the remaining syrup. Lower the speed to medium, add the gelatin mixture, and beat on medium speed for 2 minutes. Decrease the speed to low and continue beating until the bottom of the bowl is no longer warm to the touch, about 10 minutes.
Use a large balloon whisk or silicone spatula to fold one-third of the meringue into the lemon cream. Repeat twice more until all of the meringue is folded into the lemon cream.
Split the cake
Unmold the cake according to the recipe at right. Spread two 3-foot-long pieces of parchment or waxed paper on the counter. Position the cake so the top is facing up. Using a long serrated knife, remove and discard the brown top crust. Turn the cake bottom up and split it into 4 even layers. After cutting each layer, use two spatulas to lift a layer off the cake and put it on the parchment or waxed paper. Arrange the layers in the order you cut them so it’s easy to assemble the cake.
Assemble the cake
Lightly oil the inside of a clean 10-inch (16-cup) 2-piece metal tube pan.
Spread one-quarter of the filling on the bottom of the pan. Place the smallest cake ring on top of the filling. Spread about one-third of the remaining lemon filling on top. Top with the next cake layer. Spread on half of the remaining filling. Repeat with the third cake layer and remaining filling. Top with the last cake layer and lightly press it down. Cover tightly with plastic wrap and refrigerate for at least 12 hours or overnight.
To unmold, wet a kitchen towel under very hot water and wring out the excess. Wipe the sides and bottom of the pan to help release the cake smoothly.
Set the pan on top of a canister that’s smaller than the pan’s removable bottom and higher than the pan’s sides, and gently press down on the sides of the pan. If it doesn’t slide down easily, apply more heat to the sides.
Run a long offset spatula between the bottom of the cake and the pan. Run a wire cake tester or wooden skewer around the inner tube. Invert the cake onto a serving plate and remove the tube portion of the pan. Slice and serve the cake.