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.

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.

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.

Photograph by Levi Brown