To make the sauce, combine lemon juice, lemon rind, brown sugar, golden syrup, butter and 375ml (1 ½ cups) water in a large saucepan. Stir over a low heat until melted.
Meanwhile, combine the flour and custard powder in a large bowl. Use your fingertips to rub the butter into the flour mixture until it resembles fine crumbs. Stir in the caster sugar and lemon rind. Make a well in the centre. Pour in the milk. Use a flat bladed knife to mix until just combined.
Spoon dessertspoonfuls (about 16) of the dough into the sauce. Reduce heat to low. Cover and simmer for 15-20 minutes or until a skewer inserted into the centre of the dumplings comes out clean. Divide the dumplings among serving bowls. Spoon over the sauce and serve immediately with a dollop of custard and some extra grated lemon rind, if using.
