Step 1

Make the lemon curd: Whisk the sugar, egg yolks, lemon juice, lemon zest, cornstarch and salt in a small saucepan over medium-low heat; cook, whisking occasionally, until the sugar dissolves. Continue cooking, whisking constantly, until thick, about 10 minutes. Whisk in the butter a few pieces at a time until melted, then strain through a fine-mesh sieve into a bowl. Press plastic wrap directly onto the surface and refrigerate until set, at least 2 hours.

Step 2

Make the meringue: Whisk the egg whites and sugar in a heatproof bowl set over a saucepan of simmering water (do not let the bowl touch the water) until the mixture is warm and the sugar dissolves, 3 to 4 minutes. Remove the bowl from the pan; add the cream of tartar and vanilla and beat with a mixer on medium-high speed until stiff, glossy peaks form, 6 to 8 minutes.

Step 3

Assemble the cake: Put 1 cake layer on a platter; spread the lemon curd on top, stopping about 1/2 inch from the edge. Top with the second cake layer and press down gently. Cover the cake with the meringue, using the back of a spoon to make swirly peaks. Brown the meringue with a kitchen torch. Serve immediately or refrigerate up to 1 day.

Step 4

Preheat the oven to 350 degrees F. Butter two 9-inch-round cake pans and line the bottoms with parchment paper; butter the parchment and dust the pans with flour, tapping out the excess.

Step 5

Whisk 3 cups flour, the baking powder and salt in a bowl until combined. Beat 2 sticks butter and the sugar in a large bowl with a mixer on medium-high speed until light and fluffy, about 3 minutes. Reduce the mixer speed to medium; beat in the eggs, one at a time, scraping down the bowl as needed. Beat in the vanilla. (The mixture may look separated at this point.)

Step 6

Mix 1/2 cup water with the cream in a liquid measuring cup or bowl. Beat the flour mixture into the butter mixture in 3 batches, alternating with the cream mixture, beginning and ending with flour, until just smooth.

Step 7

Divide the batter between the prepared pans. Bake until the cakes are lightly golden on top and the centers spring back when pressed, 25 to 30 minutes. Transfer to racks and let cool 10 minutes, then run a knife around the edge of the pans and turn the cakes out onto the racks to cool completely. Remove the parchment. Trim the tops of the cakes with a long serrated knife to make them level, if desired.