Preheat the oven to 180°C or 160°C fan-forced. Grease and line base and side of a 20cm round cake pan.

Beat butter, sugar and vanilla bean paste in a medium bowl with an electric mixer until pale and creamy. On a low speed, add the beaten egg, a little bit at a time, alternating with a spoonful of flour. Gradually add the milk, then the remaining flour and salt.

Spread the batter into the prepared pan; smooth top. Bake for 50-60 minutes or cooked when tested (a skewer inserted into the centre should come out clean).

Meanwhile, using a zester, remove the rind from the citrus fruits and squeeze juice. Combine the orange, lemon and lime juices. and the caster sugar in a medium saucepan. Simmer over a medium heat for 20 minutes or until the sugar is dissolved and a syrup has formed. Add the zests and simmer for 1 minute, then remove from the heat.

Remove the cake from the oven. Leave the cake in the pan and immediately pierce it all over with a skewer. Pour most of the syrup over the cake, reserving 1/2 cup (l25ml) to serve.

When cake has cooled, transfer to a serving plate and spoon over the remaining syrup.