Butter a 1.2 litre heatproof bowl and pour golden syrup into the bottom.
Whisk butter and sugar until fluffy and stir in lemon zest and vanilla seeds.
Add eggs one at a time, then fold in flour and baking powder to form a batter.
Spoon mixture into the bowl and cover with pleated greaseproof paper and foil secured with string.
Place a saucer in a large saucepan, set the bowl on top, and add water until two-thirds up the side.
Bring to a boil then simmer for 60 to 90 minutes.
Whisk egg yolks and sugar in a bowl until blended.
Bring milk, cream, and vanilla to a boil, then pour over eggs while whisking.
Heat mixture slowly until thickened enough to coat a spoon and strain through a sieve.
Invert the pudding onto a plate, slice, and serve with warm custard.