Grease a 900ml pudding bowl. Melt butter with milk and allow to cool to tepid.
Stir in the sour cream and then whisk in the egg and sugar, continue whisking until smooth.
Sift together the dry ingredients.
Make a well in the centre of the flour mixture and pour in the liquid, stirring until smooth. Add the stem ginger.
Put mixture into pudding bowl and cover with 2 layers of buttered greaseproof paper. Secure with string.
Steam for 1½ hours.
To make the sauce, measure all the ingredients into a saucepan and heat gently until the sugar has dissolved. Bring to the boil for about 5 minutes until pale, golden and still runny. Allow to cool. The sauce will be of a coating consisitency and may be kept in the fridge.
Turn the pudding out onto a serving plate and serve with the sauce.