Step 1
Line the base of a 20cm cake tin with baking paper. Leave the sides ungreased and unlined. Preheat the oven to 165C fan-forced (185C conventional).
Step 2
Melt the butter in a small pan over low heat (or in the microwave), and set aside to cool to tepid.
Step 3
Place the egg yolks in the bowl of an electric mixer, add 100g of the sugar, salt and vanilla. With the whisk attachment on medium-high speed, beat until thick and pale (about 3-4 minutes). Scrape down the bowl, then, on low speed, add the hot water. Once incorporated, gradually increase the speed to medium-high and beat until thick and pale (another 3-4 minutes). Remove the bowl from the mixer and scrape the mix into a large, wide mixing bowl.
Step 4
Add half of the melted butter to the bowl and sift in half of the flour, then fold gently with a hand whisk (a method used with light batters to incorporate ingredients swiftly and airily) until almost combined. Repeat with the remaining butter and flour, folding through until almost incorporated (it doesn’t need to be fully combined yet).
Step 5
Place the egg whites in the clean bowl of the electric mixer and beat with the whisk attachment on medium-high speed. When frothy, add the remaining 50g caster sugar and keep whisking until firm peaks form.
Step 6
Add about one third of the egg-white mixture to the egg-yolk batter and fold gently to combine with a hand whisk. Fold in the remaining egg whites.
Step 7
Scrape the mix into the prepared cake tin and place in the oven. Bake for about 35 minutes or until the cake is a light golden-brown on top and a skewer inserted into the middle of the cake comes out clean.
Step 8
Remove the cake from the oven and immediately drop the tin on the kitchen bench from about a 20-centimetre height. This feels counterintuitive, but it helps minimise the “sinking” that often happens with airy cakes. Place the cake, still in its tin, on a wire rack to cool completely.
