For the marmalade, grease a 12cm/5in diameter ovenproof glass bowl with butter.
Combine the butter and sugar in a blender or food processor until pale and creamy.
Add the flour and eggs, milk and two tablespoons of the marmalade and blend again until combined.
Spoon the remaining two tablespoons of marmalade into the base of the greased bowl. Add the batter.
Cover with cling film and cook on full power in the microwave for 4-5 minutes until the cake is set and a skewer comes out clean.
For the custard, heat the milk and vanilla pod in a saucepan, until it just reaches simmering point. Remove from heat.
Whisk together the egg yolks, sugar and cornflour in a bowl.
Remove the vanilla pod from the milk and pour the milk into the bowl with the eggs, whisk continuously until the mixture forms a smooth liquid.
Return the mixture to the saucepan over a medium heat, stir constantly until the custard thickens enough to coat the back of a spoon. Take off the heat immediately and pour into a jug.
To serve, turn the sponge out onto a plate and pour over the custard.