1.Preheat oven to 180°C/350°F. Grease a 20cm x 30cm rectangular slice pan; line base with baking paper, extending the paper 5cm over long sides.
2.Beat egg yolks and caster sugar in a small bowl with an electric mixer until thick and creamy.
3.Beat egg whites in another small bowl with electric mixer until firm peaks form. Fold one-third of the egg white into egg yolk mixture; fold in remaining egg white.
4.Sift flour twice. Sift flour over egg mixture; fold in gently until well combined. Pour mixture into pan.
5.Bake cake 15 minutes. Stand in pan 5 minutes. Use a skewer to pierce holes all over cake.
6.Combine milks and rum in a jug; carefully pour half the mixture over cake in pan. When most of the milk mixture is absorbed, add remaining mixture. Cover; refrigerate overnight.
7.Score a 1.5cm border around the cake. Turn cake, top-side up, onto a serving platter. Spread cake with half the lemon curd.
8.Beat cream in a small bowl with electric mixer until soft peaks form; spread on curd. Dollop spoonfuls of remaining curd on cream; swirl for a marbled effect. Dust cake edges with sifted icing sugar.