- Preheat oven to 175C/ 350F. Grease and line a 28cm heart-shaped springform pan. Set aside. Melt butter over low heat. Pour into small mixing bowl and set nearby.
- Beat eggs, caster sugar, extract and salt in the bowl of stand mixer about 5-6 minutes on high speed until the mixture is lemon white in colour and triples in volume. If you beat the mixture with a hand-held electric mixer, then set the mixing bowl over a pan of simmering water at about 40C/105F. The base of the bowl should not touch the hot water. After the mixture has become thick and pale, remove the bowl and continue beating for 2-3 minutes until the mixture is cool.
- Fold the sifted flour into mixture, one third at a time, just until incorporated. Pour about 1 cup of batter into the melted butter, and fold just until combined. Return butter mixture to reserved batter, and fold to combine.
- Pour batter into prepared pan and smooth batter evenly. Bake in preheated oven for 25-30 minutes or until top springs back slightly when lightly touched. Run a table knife blade around the outside edge of cake, freeing the sides and allowing air to get under the layer. Invert cake onto rack and allow it to cool completely.