Butter and sugar a 1.2 liter heatproof bowl, then pour in the mixed treacle and golden syrup.
Whisk butter and sugar in a separate bowl until light and fluffy.
Add eggs one at a time, beating thoroughly after each addition.
Fold in the flour and baking powder to create a smooth, thick batter.
Spoon the batter into the prepared bowl and cover with greaseproof paper secured with string.
Cover the bowl again with foil, rolling the edges to seal.
Place an inverted saucer in the bottom of a large saucepan.
Set the bowl on the saucer and add water to the pan until it reaches two-thirds up the bowl side.
Cover the pan with a lid and bring the water to a boil.
Reduce heat and simmer for 90 minutes.
Whisk egg yolks and sugar together for the custard until pale.
Boil milk, cream, vanilla pod, and seeds in a saucepan and simmer for one minute.
Pour the hot milk mixture over the eggs while whisking constantly.
Heat the mixture slowly in the pan until thickened, then strain through a sieve.
Remove the pudding from the pan and discard the foil and paper.
Turn the pudding out onto a serving plate.
Slice the sponge and serve with the warm custard.