Butter a 1 litre pudding bowl and put pecan halves in bottom (a metal 'bombe' mould gives a really dramatic shape, but any pudding bowl will do.)
Put castor sugar and water into a small saucepan, bring to a boil over moderate heat, cover and simmer for 5 minutes.
Uncover, increase heat and boil until it turns a deep caramel colour.
Remove from heat and immediately stir in cream and butter. If there are any clumps of caramel, return to a low heat and stir until smooth. Pour over nuts in a pudding bowl.
Cream butter and soft brown sugar together until light and fluffy. Beat in eggs one at a time. Gently fold in flour and nuts, using a metal spoon. Spoon sponge mixture over nuts.
Cover with buttered greaseproof paper, then foil and tie with string. Put into a large saucepan on top of a folded napkin or saucer to keep bowl off bottom of pan and pour in boiling water to come halfway up sides of bowl.
Cover and simmer gently for at least 2 1/2 hours (it takes this time to cook, but the beauty of steaming is that it can go longer without spoiling or drying out).
Turn out onto a serving dish and serve with bourbon sauce