Butter a 1.5-litre pudding basin. To make the sponge, cream the butter and sugar in a mixing bowl until pale and fluffy. Add the eggs, one at a time, mixing well between each addition, then add the vanilla extract.
Sift the flour and baking powder into the bowl and fold into the sponge mixture,
ensuring there are no lumps of flour remaining. Finally, stir the milk into the mixture.
Fill the basin with the sponge mixture and cover with a double layer of buttered foil and baking parchment, making a pleat in the centre to allow the pudding to rise. Tie the foil securely with string, then place in a steamer or large pan containing enough gently simmering water to come halfway up the sides of the basin. Steam for 2 hrs until an inserted skewer comes out clean.
To make the custard, bring the milk to the boil. In a bowl, beat together the yolks, sugar, cornflour and vanilla extract. Pour the scalded milk over the egg mix and beat well. Return to the pan and place over a low heat, stirring for a few mins until the mix begins to thicken. Take off the heat and leave to cool.
For the butterscotch sauce, dissolve the sugar with 2 tbsp water over a low heat. Bring to the boil; do not stir. When the sugar has turned into a dark, golden caramel, whisk in the butter. Remove from the heat and stir the cream through.
When the sponge is cooked, turn out onto a plate, drizzle with the butterscotch sauce and serve with the custard.