Preheat the oven to 200C/400F/Gas 6. Grease a 25cm/10in pie dish with butter.
For the gooseberry pie, heat the gooseberries and sugar in a pan over a low to medium heat for 6-8 minutes, or until softened. Set aside to cool.
Cut the shortcrust pastry in half and roll each piece out onto a lightly floured work surface, until they are 5-8cm/2-3in larger than the diameter of the pie plate.
Line the prepared pie plate with one of the rolled pastry sheets and trim the edges. Spoon the gooseberry and sugar mixture into the pie case in an even layer, leaving 2.5cm/1in free around the edges.
Lay the remaining pastry sheet over the pie plate and trim off any excess. Seal the edges of the pastry using a fork. Prick the pie all over with a fork and brush all over with the beaten egg.
Transfer the gooseberry pie to the oven and bake for 25-30 minutes, or until golden-brown.
Meanwhile, for the champagne custard, in bowl, whisk together the egg yolks and sugar until pale and fluffy.
Heat the milk, cream, champagne and vanilla seeds in a pan until simmering but not boiling. When the mixture is almost boiling, remove the pan from the heat and gradually pour it over the egg and sugar mixture in a thin stream, whisking continuously.
Return the custard mixture to a clean pan and heat over a low to medium heat for 8-10 minutes, stirring continuously, until the mixture is thick enough to coat the back of a spoon.
To serve, slice the pie into 6-8 wedges. Place one wedge into the centre of each serving plate and pour over the champagne custard.
