Grease a 12cm diameter ovenproof glass bowl with butter.
Blend butter and sugar until pale and creamy.
Add flour, eggs, milk, and 2 tablespoons of marmalade to the blender and combine.
Spread the remaining 2 tablespoons of marmalade in the bowl base and add the batter.
Cover with cling film and microwave for 4-5 minutes until set.
Heat milk and vanilla in a saucepan until simmering.
Whisk egg yolks, sugar, and cornflour in a separate bowl.
Remove the vanilla pod and whisk the hot milk into the egg mixture.
Return the mixture to the saucepan and stir over medium heat until thickened.
Turn the sponge out onto a plate and serve with the hot custard.