Step 1

For lemon curd, whisk eggs, yolk and sugar in a heatproof bowl to just combine, then whisk in juice and rind. Place bowl over a saucepan of simmering water (do not allow the water to boil) and lightly whisk mixture to stop curd cooking too quickly around the edge of the bowl until it reaches 85C on a sugar thermometer or thickens to the consistency of whipped cream (25-30 minutes). Cool until just tepid (40 minutes), then add butter a little at a time, whisking well between additions. Cover with plastic wrap and refrigerate to chill and set (at least 4 hours or overnight).

Step 2

Preheat oven to 160C. Line two 22cm-diameter springform cake tins with baking paper. Beat butter, sugar and vanilla in an electric mixer until pale and fluffy (4-5 minutes). Gradually add egg, beating between additions and scraping down sides of bowl, until all incorporated.

Step 3

Sieve dry ingredients into a bowl, then add to egg mixture in 2 batches, folding between additions. Add milk and fold to combine, then divide evenly among prepared tins and set aside.

Step 4

For meringue, whisk eggwhites and a pinch of salt in an electric mixer on medium speed until mixture forms a soft ribbon (3-4 minutes), then gradually add sugar, whisking until meringue is glossy (3-4 minutes). Dot half evenly over each cake, and swirl the meringue on one for decorative effect for the top layer, then bake both, turning halfway, until the meringue is slightly golden (30 minutes; it's impossible to test if the sponge is cooked because of the meringue). Cool completely in tins (3 hours).

Step 5

Remove cakes from tins and carefully peel baking paper off. Spread lemon curd evenly over bottom cake (serve any remaining with the cake), top with whipped cream, then place second cake on top. Dust with icing sugar and serve.